Firefox (ABC Sunday at 8 p.m.) is...

Firefox (ABC Sunday at 8 p.m.) is the only time Clint Eastwood the star has not been well-served by Clint Eastwood the director, who allowed Craig Thomas’ taut novel to sag and linger in its adaptation to the screen. Eastwood is an Air Force major severely traumatized by his Vietnam experiences yet he is improbably recruited for an impossible mission: snatch the Russians’ newest, most magical MiG. For all its design dazzlement, Firefox has engine trouble at its most crucial point, the script.

Airing on Sunday at 6 p.m. are Mel Frank’s lively comedy-Western The Duchess and the Dirtwater Fox (Channel 5), with Goldie Hawn and George Segal in the title roles, and Don Siegel’s grandly elegiac Western The Shootist (Channel 13), which marked an apt farewell to the screen for John Wayne.

Always a fine, intuitive actor, Nick Nolte had two of his best roles in Cannery Row and Under Fire, but wouldn’t you know they screen opposite each other, on Channel 5 and Channel 13, respectively, on Tuesday at 8 p.m. and again Saturday at 8 p.m. In the first, he’s John Steinbeck’s Doc, a dedicated marine biologist who’s also a soft touch. In the second, he’s a free-lance photographer caught in a love triangle (vying with Gene Hackman for Joanna Cassidy) and the moral dilemma of maintaining objectivity while covering the last days of Somoza’s dictatorship in Nicaragua.

Wednesday at 8 p.m. brings two of the ‘60s best genre movies, Don Siegel’s tough cop flick Madigan (Channel 5), with Richard Widmark and Henry Fonda, and John Boorman’s stylish thriller Point Blank (Channel 13), with Lee Marvin and Angie Dickinson.

In the new TV movie Sin of Innocence (CBS Wednesday at 9 p.m.) Megan Follows and Dermot Mulroney are a stepsister and stepbrother who fall in love; it all happens when Follows’ mother (Dee Wallace Stone) and Mulroney’s father (Bill Bixby) marry.

The struggle to adjust and readjust after a broken marriage is the theme of Paul Mazursky’s impressive An Unmarried Woman (Channel 5 Thursday at 8 p.m.). How Jill Clayburgh copes after her stockbroker husband Michael Murphy leaves her for a sweet young thing is what the film is all about. Alan Bates is the romantic painter Clayburgh meets at just the right moment.

The Sound of Music, one of the most beloved movies of all time, is repeated on NBC Friday at 8 p.m. With Julie Andrews as the star, this Rodgers and Hammerstein musical about the Von Trapp Family singers is a visual and vocal delight.

National Lampoon’s Vacation (CBS Friday at 9 p.m.) runs out of gas before it gets under way. Chevy Chase stars as an All-American idiot from suburban Chicago who hits the road with his wife (Beverly D’Angelo) and their two kids.

Milos Forman’s The Fireman’s Ball (Channel 28 Saturday at 10 p.m.), a 1967 best foreign film Oscar nominee, is a lovingly observant account of the human comedy, set at the annual shindig put on by the volunteer firemen in a rural Czech town. Also airing at 10 p.m. (on Channel 9) is Conduct Unbecoming, an engrossing if stagy film that revealed a rancid stew of emotions that lurked beneath the teatime tidiness and clockwork pomps of regimental frontier life in British India, 1878.

Selected evening cable fare: Stevie (AE Sunday at 6 & 10); Beyond the Limit (Z Monday at 9); Conversation Piece (Z Tuesday at 9); Nijinsky (Bravo Wednesday at 8:30); City of Women (Z Thursday at 9); Ticket to Heaven (Lifetime Friday at 8); Lola Montes (Bravo Saturday at 9).