As the networks narrow their choices for the fall schedules (to be announced within the next few weeks), one of the difficult calls for each is the marginally successful series . . .

Such programs have a couple of promising ingredients and a recognizable core audience. Each has been stuck with a tough time period in the season that ended last night . . .

In this category, series like CBS' "Equalizer," ABC's "Spenser: For Hire" and even the latter's "Mr. Sunshine" come to mind . . .

In the case of "MacGyver," ABC's decision is becoming easier by the week . . .

"MacGyver," which features Richard Dean Anderson as not-quite-a-superhero who relies on science instead of brute strength to get out of trouble, started off the 1985-86 season very nearly dead . . .

He was on Sunday night against CBS' "Murder, She Wrote" and that was just about all she wrote . . .

In that spot, he climbed to 46th place in the ratings one week. At another network he'd have been gone . . .

But Mac was with third-place ABC, he was expensive to produce, and in mid-January the network moved him to 8 p.m. Wednesday, where NBC's "Highway to Heaven" was already established as a favorite of women over 55 in rural markets . . .

It was easy for "MacGyver" to carve second place for himself in that time slot when the competition on CBS was "Mary" and "Foley Square," two of the larger losers this second half of the season . . .

But on March 19, "MacGyver" beat a Bob Hope special on NBC and finished 22nd. Subsequently, he's been taking on "Highway to Heaven" reruns and has beaten them two weeks in a row to take the hour . . .

Last week, "MacGyver" won again with a 15.7 Nielsen rating and a 25 percent audience share, beating a brand-new circus special on CBS, which had a 15.3/24, and a "Highway" rerun at 14.7/23 . . . ABC's tough Wednesday schedule is suddenly that much tougher . . .

Mac even has Captain Airwaves, a "Highway" fan from the start, watching and liking his show, for gosh sakes . . .

And apparently already headed for a fall tryout on ABC -- the network has ordered six shows without a pilot -- is a spinoff of the movie "Gung Ho" . . .

It will star Ned Eisenberg in the Michael Keaton role plus Gedde Watanabe and much of the Japanese cast seen in the movie as managers of a Japanese auto manufacturer that sets up shop in a closed-down Pennsylvania automobile plant . . .

Network news divisions are anticipating big numbers tomorrow when the weekly ratings come out, after a week of the Libya crisis . . .

Ditto ABC's "Nightline," which two weeks ago beat NBC's Johnny Carson for first place (it aired four nights vs. five times for "The Tonight Show" and the CBS lineups that week), its first weekly victory since last October during the Achille Lauro hijacking . . . Also in the News

From our These Guys Are Serious file: The big news in New York last week was the firing by WABC of l8-year anchor Roger Grimsby, who was pulled so fast he didn't even get a chance to say goodbye on the 6 o'clock Thursday night news he coanchored with Bill Beutel. . .

Grimsby was reportedly paid $900,000 a year and has received $1 million-plus for the 14 months remaining on his contract . . .

A top prospect to replace Grimsby is Mary Nissenson, who covered national politics for NBC out of the Chicago bureau in 1984 before jumping to WABC . . .

Although Grimsby's battles with News Director William Applegate have been cited as a reason, our sources at ABC suspect the departure was another budget-cutting move by Cap Cities/ABC Inc., which has been paring the staffs of the four remaining ABC-owned stations, including WABC, since the merger went through in January . . .

Also last week, Cap Cities/ABC approached its affiliates' board of governors with a proposal to cut back on the fees the network pays affiliates to carry its programs . . .

To make up the difference, ABC would give each local station more advertising time in each hour . . . the additional time to come out of the program contents, which run only about 23 minutes every half hour as it is (just how that squares with the voluntary rule among broadcasters to keep commercials-per-hour at current levels was not made clear) . . .

Sports programming, including Monday Night Football, would be one of first to be tested . . .

The affiliates board was reported to be unenthusiastic about the proposal . . . which is expected to be brought up again in June, when all the ABC affiliates meet out in Los Angeles . . .

Due to appear on the Thursday telecast of "Star Search" (Channel 5 at 8 p.m.) is 10-year-old Demetri Callas, son of Zandra Zarvis of Waldorf, Md. . . .

The fifth-grader at J.P. Ryon Elementary School will compete in the actor category . . . Moving Right Along

ABC News cameraman Ken Blaylock has been reelected to a third consecutive term as president of the White House News Photographers Association. . . .

When they start handing out Emmys for yesterday morning's eloquent "Horowitz in Moscow" special on CBS' "Sunday Morning," let's hope they save a mention anyway for sponsor AT&T, which used only six minutes of its allotted 24 minutes for commercials . . .

And from our So That's How They Got The Scoop file: Captain Airwaves, as is his wont, wiggled the ole rabbit ears in tribute Thursday night as "Entertainment Tonight" revealed that Patrick Duffy will get $75,000 per episode and that he will definitely play somebody other than Bobby Ewing when he returns to "Dallas" . . .

Turns out Duffy had been signed to host an upcoming "Entertainment This Week" . . .

And from our But Can He Slam Dunk? file: when CBS Entertainment notified CBS affiliates of the new/old "Dallas" cast member, the telex identified him as . . . Patrick Ewing . . .

Ted Turner, president of the Turner Broadcasting System, will speak at George Washington University this afternoon on "Television News: Past, Present, Future" . . .

The 4:30 talk in the Marvin Center at 21st and H streets NW is open to the public . . .

Jack Loftus, who worked for CBS Inc. here a few years back and most recently has been TV and radio editor for Variety (where he had fun taking a few whacks at CBS Inc.), has been named vice president, corporate communications, for the Madison Square Garden Corp. in New York . . .

Producer Linda Yellen took a lot of heat five years ago defending her choice of Vanessa Redgrave to play the lead in "Playing for Time" . . . the CBS movie based on the memoirs of concentration camp survivor Fania Fenelon . . . despite Redgrave's anti-Israel stands . . .

Yellen recently cast Redgrave again as transsexual tennis player Renee Richards in a TV movie called "Second Serve," which will air soon on NBC . . .

A couple of weeks ago, Redgrave and 37 other actors signed a petition asking England's Council of Actors Equity to request its members not to accept work in Israel . . . and reportedly backed an attempt to deny to Israel the use of completed TV and film productions involving Equity players . . .

The motion was defeated last week, but Yellen is incensed. She told the L.A. Times, "I will never work with Vanessa Redgrave again. I feel absolutely betrayed" . . .

"Passing [the resolution] is not the issue," said Yellen. "It is just abhorrent in this age, where terrorism is a daily reality, [that she] is using the forum she developed as an actress to spout these hateful political ideas" . . .

She said she has always found Redgrave's political views "repugnant," but had supported her in the past on grounds of artistic and political freedom . . .

"But here," said Yellen, "she is saying that others aren't entitled to the same freedoms" . . .

Because the networks would either not run commercials for the movie at all -- or else only late at night -- and because some newspapers wouldn't take the ads for it either, Tri-Star Pictures has changed the name of "Sexual Perversity in Chicago," based on a David Mamet play, to "About Last Night . . ."

The movie, and presumably the TV commercials, will be released this summer . . .