In "Old Boyfriends," director Joan Tewkesbury and a talented cast end up hostages to an unsalvageable script. Best known as the screenwriter on Robert Altman's "Nashville," Tewkesbury demonstrates a rapport with actors and a flair for intimacy that could pay off with less counterfeit material.

The movie gets off to a flying start: The credits are superimposed over a long helicopter shot of a car speeding through traffic and smashing through roadblocks into a stone wall.The intrigue thickens when Talia Shire, projecting her customary grim mousiness, dials someone up and plays "You Belong To Me" into the receiver when a man answers.

The mystery begins to clear up-and warning signals start going off in your head-as soon as Shire's character gets confidential on the sound-track and outlines the plot. She tells us that she's been reading her old diaries over and over. This bad ideas has inspired a worse one: Now obsessed by recollections of her old boyfriends, she has decided to look them up. She explains herself in terms drippily narcissistic: "I realized if I could figure out why I loved them then, I could figure out myself and love myself."

Her sentimental journey turns out to be progrssively regressive. She pays her first call on college flame Richard Jordan, now working as a nontheatrical filmmaker in Denver, then goes on to get even with the high school lowlife who once humiliated her, John Belushi, and attempt imprompty sex therapy on a mentally disturbed boy, Keith Carradine, the kid brother of her preteen sweetheart who has perished in Vietnam.

Each episode reinforces the unsympathetic impression created by the heroine's original statement of her aim. Ostensibly seeking the key to Love of Self, she repeatedly humiliates herself and the "boyfriends." Curiously, Jordan appears to be the only legitimate boyfriend in her past. He's also so attractive-good-looking, good-natured, sane and unpretentious-that it's impossible to believe the heroine's claim that she thrice rejected his marriage proposal.

Jordan has put on a little weight and let his hair straggle. These details are in character, and they enhance his likability. A plump, unkempt look takes the edge off his smoothly handsome features, and this casually irresistible performance in a better movie would have made him a star at last. As it is you want him to run for cover when she turns up begging to be loved.

After Jordan rejects her desperate, out-of-the-blue marriage proposal, Shire scurries away to make trouble for the Belushi character, a poor slob still mired in his hometown, where he runs a tuxedo rental business by day and an amateur rock 'n' roll band by night. The heroine is supposed to be justified in degrading him, since he tattled on her after a backseat grope during their senior year.

Belushi's witty performance makes a shambles of this intention, at best a lame excuse for softcore comic relief.For one thing, Belushi is too entertaining to be resented.When he goes into his act, mimicking standard renditions of "Let's Rock" and "You Belong To Me," one wishes Twekesbury would junk those reaction shots of Talia Shire contemplating revenge. Better to just concentrate on that extraordinary comedian as he characterizes a third-rate talent trying to sustain a sense of self-importance. Belushi makes this loser's underlying melancholy so apparent and oddly affecting that the heroine seems cruel for wanting to get even. Can't she see how out of it he is?

While Shire is busy seducing the simpleminded Carradine character, a worried Jordan tries to locate her. By this time we've learned that she's a clinical psychologist. Jordan turns up the revelation that she flaked out after her marriage collapsed. The peculiarly confused, hypocritical tendencies of the screenplay are resolved with an absurdly upbeat ending. While Shire must sit still for a stern paternal scolding from John Houseman, cast as Carradine's shrink, she is also let off the hook rather blithely, in time to look forward to a brighter tomorrow as Jordan's ever-lovin' headache.

But neither Shire not Tewkesbury can bring the morose little wretch to credible life. The character is the invention of male screenwriters, Paul and Leonard Schrader, and her neurotic excesses may owe more to their anxieties and resentments than anything else.

Nearly everyone else is interesting in some respect: Gerritt Graham as a pompous actor, William Bassett as the heroine's ex-husband, Bethel Leslie as Carradine's edgy mother, Nina Jordan as her father's sensible teen-age daughter and Buck Henry and P.J. Soles as a private investigator and his sexy secretary, encountered moments after some kind of spat.

With so many lively and amusing possibilities on the periphery, it seems a pity that "Old Boyfriends" should be doomed by a vacuous center. CAPTION: Picture, Talia Shire and John Belushi in "Old Boyfriends"