Sylvester Stallone could use a muscle relaxant about now. Rambo III, which was to begin shooting this month, has been postponed until the fall. According to an insider, Sly, bent out of shape by the box office failure of Cobra and Over the Top, is terrified of a third flop. The only things he's been shooting down these days are Rambo III scripts. Says one friend: "He's paranoid now."

The scene of Gary Hart's notorious rendezvous with Donna Rice may be on the block, back door and all. There's no "For Sale" sign on Hart's two-bedroom Capitol Hill town house. But Hart's realtor, Terri Robinson, an old friend of Lee Hart's, says that she's received "plenty of phone calls" about the house, which has become something of a tourist attraction since the infamous May weekend. "If they get a good offer, I can't say they would turn it down," says Robinson. She would only hint at the reason: "It's a tough situation for everybody, as you can imagine." The house, which she estimates would sell in the mid-$300,000 range, also has three stories: his, hers and the Miami Herald's.

Guess which television couple is next in line to catch fire after seasons of smoldering passion? On Who's the Boss? next season, Angela (Judith Light) goes to the shrink and, through the miracle of therapy, discovers that it's really Tony (Tony Danza) whom she loves. Off-camera and for much less than $100 an hour, there are those in the studio who could tell Judith that she is being too patient with the temperamental actor. According to insiders, Light is the peacemaking pro who makes things bearable when Danza launches into a tantrum. "People are afraid of his mood swings," says one source. "In Judith's professional, solid way she holds the show together." In other words, she's the boss.

Larry Hagman's anti-smoking tactics can be a pain to some. Hagman carries battery-operated fans, which he pulls out at the drop of a match and aims at any cigarette smoker who crosses his path. But Neiman-Marcus loves the idea and wants to feature mini-fans like his in its 1987 Christmas catalog. The fans would come in solid gold and sterling silver cases.

Platoon's Charlie Sheen has entered into another battle. Screaming "I can't take it no more!" Sheen joins the heavy-metal band Urgent in a rock video that shows the tortures of love. For this appearance, he wears a tank top and moans and groans about rejection from a gorgeous young female rocker. Heavy metal is hell.

Heaven is good credit. That's what actor-director Robert Townsend has now that Hollywood Shuffle, the film he made with a prayer and several credit-card cash advances, has proved to be a hit. On a flight to New York, Townsend heard a song by the Jamaica Boys on his headphones and choreographed a video for the group on an airline napkin. The video has been sent to MTV for possible airing.

Bill Murray hasn't been seen onscreen since his role as a disabled World War I vet in 1984's Razor's Edge led to cutting reviews and bad box office. But Babaloo Mandel and Lowell Ganz (they co-wrote Night Shift) are preparing a happy return to the screen for him. In their comedy, to be directed by Sydney (Tootsie) Pollack, Murray inherits a ballet company and with it none other than Mikhail Baryshnikov.

Boys and girls! It's Howdy Doody time! Again! Buffalo Bob, Clarabell, Princess Summerfall Winterspring and that slightly wooden star Howdy Doody will have a 40th-birthday-party reunion in late November that will be syndicated on TV across the country. Guests lining up to hoot it up in the peanut gallery are Pee-wee Herman, Pat Sajak, Vanna White, Garrett Morris, Lou Ferrigno and Phyllis Diller. So, boys and girls! What time is it?