* Actor Robert Downey Jr., who's already served six months behind bars, was thrown in jail again yesterday after acknowledging to a California judge that he's still doing illegal drugs and violating his probation, the Associated Press reports. Superior Court Judge Lawrence J. Mira ordered him to the jail's drug rehab unit pending an Aug. 5 hearing.

* The more things change, the more they stay the same: Onetime Democratic presidential nominee Michael Dukakis has phoned from Boston to quibble with last week's report that he's headlining a Washington fund-raising dinner for the Library and Archives of New Hampshire's Political Tradition. "I have no idea what this is dinner is and I have no plans to be there," the former Massachusetts governor told us from Northeastern University, where he teaches public management and policy--never mind that the dinner organizers insist he confirmed his acceptance weeks ago. "When you called, I wondered, what is this cryptic message about a dinner? Am I hallucinating?"

* When a White House press bus was mired in the Macedonian mud yesterday during President Clinton's visit to a refugee camp, Sam Donaldson used some elbow grease, reports The Post's Charles Babington. The ABC News personality, notwithstanding his nice suit, got out and began pushing. But after a few minutes he gave up. "We are in the hands of a stupid person," he announced, presumably referring to the driver.

* Kiss-and-tell writer Joyce Maynard auctioned 14 love letters written to her by J.D. Salinger for $156,500, the AP reports. The buyer, computer software mogul Peter Norton, says he may return the letters to the reclusive novelist.

Mel Passes the Pickle

* More evidence that we're all getting older: Local institution Mel Krupin, the last of a vanishing breed of Washington restaurateurs, has sold his popular delicatessen, Krupin's, to Young Turk caterer Jimmy Polsky. "Mel's worked hard for 40 years. He's tired," says the 43-year-old Polsky, who's merging his Columbia Cafe with Krupin's restaurant on Wisconsin Avenue. He paid an undisclosed sum for the business and to use the Krupin name.

Mel, who turns 70 this year, will stay on as a greeter and consultant for at least another 2 1/4 years. When we asked him what he possesses that the whippersnappers lack, Mel--known as the Don Rickles of restaurateurs--was quick with his reply: "Personality."

An Irate Imus

Our good friend Don Imus--in town to do his radio show from Rockville's WTEM-AM after accepting a heartwarming humanitarian award Monday night from the National Association of Broadcasters-- cussed out his limo driver yesterday morning when confusion developed over a ride for Imus's wife, Dierdre, and their 11-month-old son, Wyatt.

According to Sonny Henderson, owner of S and S Limousine Service in Lanham, chauffeur Eddie Robinson showed up promptly at 4:30 a.m. to pick up the shock jock for his 5:30 broadcast. But when Imus emerged from the lobby of the Four Seasons, police were detaining Robinson as a witness to the bizarre behavior of a man on the street who'd been pulling down his pants and shouting. The cops were questioning Robinson about this character, who was face down and handcuffed on the sidewalk in front of the hotel. "They had him Rodney King-style," Imus told The Post's Frank Ahrens.

The harried humanitarian got positively cranky and insisted that Robinson, a retired D.C. government employee, immediately install a child car seat and give a ride to Imus's wife and son. Frustrated and angry, Imus flagged down a cab and jumped in, only to realize that he had no idea where WTEM's studio is located. After phone calls for directions, he arrived about 10 minutes before his show was to begin.

Henderson reports that his employee said afterward: "That man is crazy."

CAPTION: Onetime candidate Dukakis: Confused again? (1988 photo)

CAPTION: Restaurateur Mel Krupin.

CAPTION: The shock jock wasn't a happy camper.