CBS Entertainment yesterday jettisoned two-thirds of its low-rated Saturday night lineup in favor of a two-hour movie rerun package devoted to Peter Ustinov as Hercule Poirot in a series of Agatha Christie mysteries ...

CBS will air the movies starting Nov. 14 at 8 into the Christmas season, when specials take over. Introduction of a new regular series schedule for Saturday won't happen until January ...

"My Sister Sam," "Everything's Relative" and the one-hour "Leg Work" will air for the last time tomorrow night. "West 57th" will remain at 10 p.m. ...

"Everything's Relative" and "Leg Work" will both go to the boneyard after tomorrow but "My Sister Sam" will remain in production for a return to the schedule at some future date ...

"Leg Work" executive producer Frank Abatemarco said late yesterday that four more episodes of the one-hour series remain and he's petitioned the network to give the program one more chance in a different time slot. Regardless, CBS sources on the West Coast say the network is high on series star Margaret Colin and she is almost sure to have another series ...

Nov. 14, CBS will introduce its Saturday movie run with Peter Ustinov as Poirot in "Agatha Christie's 'Dead Man's Folly,' " followed on Nov. 21 by "Agatha Christie's 'Thirteen at Dinner,' " and on Nov. 28 by "Agatha Christie's 'Murder in Three Acts' " ...

Up against the dynamite NBC sitcom schedule and "Hunter," Saturday night has been a disaster area for CBS this season, with the night averaging a minuscule 6.6 Nielsen rating, including a 6.6 for "West 57th" (each rating point represents 886,000 TV homes) ...

Without Saturday, according to CBS research chief David Poltrack, the network would be averaging a 14.5 after six weeks in the schedule instead of the 13.8 average that has put CBS in third so far this season ...

In the last regular season, CBS averaged a 15.8 rating to finish second behind NBC but well ahead of ABC ...

Tuesday is the other bad night for CBS so far this season, and only "Jake and the Fatman" at 9 seems certain to stick on the schedule when changes are made ...

"Right now," Poltrack said yesterday, "our Tuesday 8 to 9 is tougher than our Thursday 8 to 9 (against the NBC powerhouse hour of "The Cosby Show" and "A Different World"). We have to go up against NBC's 'Matlock' as well as two very strong ABC sitcoms ("Who's the Boss?" and "Growing Pains") ...

Poltrack is optimistic that the network can rebound and soon. All three networks are performing below last year's audience levels, which Poltrack blames on the NFL strike, a comparatively low-rated World Series on ABC and, of course, the introduction by Nielsen of its peoplemeter ratings system, which overall delivers lower viewer counts ...

He says CBS is already second in Arbitron for the key November sweeps, which began Oct. 28 for a 28-day period. The return of Agatha Christie movies is a short-term strategy to get ratings up during the sweeps ...

Last year, he said, CBS had to compete for two weeks in the fall against the World Series on NBC and a weaker network (ABC). This fall, he points out, "we competed with the World Series on ABC and a network {NBC} stronger than we are" ...

ABC, he said, faces three bad nights this season: Thursday, Friday and Saturday ...

Regarding the Saturday changes, he said the Agatha Christie movies, on past performances, are almost guaranteed to bring in double-digit ratings on Saturday nights, or nearly double the performance so far. His immediate goal is "to kill 'Facts of Life,' " the 8 p.m. NBC show that is that network's lowest rated series on Saturdays so far ...

When the November sweeps and holiday specials are out of the way, Poltrack said, a number of series are waiting in the wings as replacements for a schedule that is obviously going to see further adjustments beyond Saturday and Tuesday before the year is out ...

Both "Simon & Simon" and the sitcom "The Cavanaughs" are waiting to return to the schedule. Among the new series are Robert Conrad in "High Mountain Ranger" (a critical debacle that got very high ratings in a pilot); "Blue Skies," the story of a chap (Tom Wopat) who chucks it all and heads for a small California mill town, which Poltrack likens to "The Waltons"; and several new sitcoms, including "Trial and Error," about a pair of Hispanic brothers, one of whom is a lawyer, the other "a character"; and "Just You and Me," about a divorced producer of local TV commercials raising his 8-year-old daughter ...

Also in the News

White House Press Secretary Jim Brady received a standing ovation from the "Donahue" audience in Stamford, Conn., yesterday when he walked on the stage at the beginning of the taping (the program will air on Channel 9 at 9 a.m. Monday) ...

Part III of the miniseries "Shaka Zulu" won a 16.5 Nielsen rating and a 27 percent audience share Wednesday night on Channel 5 as the program continues to build with Washington audiences (each rating point represents 15,851 TV homes here) ...

After a considerable delay, CBS News has given an okay to a Walter Cronkite documentary, "Children of Apartheid," which the network now hopes to air soon after the end of the November ratings sweeps ...

The documentary sparked a minor internal controversy when CBS News brought in David Dison, a lawyer in a South African firm, to get his views on the government's possible response to the documentary in view of South Africa's restrictions on press coverage in the country ...

The South African government had denied a work visa to Cronkite, who entered the country under a tourist visa to tape several interviews. Similarly, the program's producer, Brian Ellis, told The New York Times he had entered the country without declaring his purpose to conduct secret interviews. Both he and Cronkite questioned the employment of an outside lawyer. CBS News pointed out that Dison's firm had previously been employed by CBS and other news organizations regarding restrictions on the press in South Africa ...

CBS News president Howard Stringer last summer also wrote to the South African Bureau of Information to reassure the government of CBS News' policy of honoring a host country's rules, following the publication in The New York Times of an article on the op-ed page by CBS News producer Richard M. Cohen criticizing cooperation by U.S. news organizations with the South African government ...

Dison subsequently reviewed the documentary. CBS accepted his suggestion that the opening line of the program, "This documentary is illegal," be dropped as a possible irritant to the government ...

The documentary has been in the works since late last year and was originally scheduled for June ...

ABC's "Good Morning America" won the early morning network race again last week with a 4.8 national Nielsen rating and a 22 percent audience share. NBC's "Today," which has fallen to second place in the race since peoplemeters were introduced in mid-September, closed the gap somewhat with a 4.6/21. CBS' "The Morning Program" had a 2.4/11, its best showing since August ...

The Public Broadcasting Service announced yesterday that it plans "an ambitious, multimillion-dollar effort to cover the '88 presidential campaign" ...

Among the projects already approved: "The Last Word," a three-hour election eve special, to be produced by WETA here and WGBH in Boston that will feature "reporting, analysis, commentary and humor" ...

On-site coverage of the Democratic and Republican conventions by "The MacNeil/Lehrer NewsHour." No word yet on gavel-to-gavel coverage, however ...

Also, "MacNeil/Lehrer" news updates on election night as well as analysis of post-convention presidential debates; a "Frontline" documentary on the campaigns; a Bill Moyers special on the presidency; and a series of 15 to 20 election spots, described as "brief bits of presidential trivia, humor and history" ...

Meanwhile, because of the summit meeting set for Dec. 7, sponsors of The Boston Globe/ ABC News Presidential Debates have canceled the events, originally scheduled for Dec. 7 and 8 for airing on PBS ...