The most erratic movie in recent memory, "The Night the Lights Went Out in Georgia," cries out for a patient, forgiving public. After getting off to a wretched start, the film settles down in mid-passage and grows unexpecedly appealing. Down the stretch it reverts to faltering form. The best policy might be to go about 30 minutes late and leave about 15 minutes early.

I know I would have walked out within the first 15 minutes if I hadn't been professionally obligated to comment on the show. The introductory sequences promise nothing but uncouth farce staged with relentless ineptitude. An irate, obese redneck storms into a motel somewhere in shabbiest Dixie, looking for his unfaithful wife, tucked away with Dennis Quaid, cast as a raffish country-and-western singer named Travis Childs, who is easily distracted by female admirers. The tumult alarms Kristy McNichol, cast as Travis' nervy kid sister Amanda, who's sleeping in an adjacent room. She hastens to pack up and help get big brother out of harm's way, presumably not for the first time.

The outlook doesn't improve in the immediate aftermath of this ugly episode, misconstrued as a rib-tickling rabble-rouser by director Ronald F. Maxwell, who was last seen making a botch of things in "Little Darlings." (Can't someone in Hollywood introduce Kristy McNichol to a few talented young directors?) Reeling from his night of dissipation, Travis excuses himself to upchuck in a phone booth, an unsavory sight gag if there ever was one. Then he and Amanda get in an altercation with a fat woman, a gratuitous encounter that suggests that the filmmakers must be embarked on a pointless crusade to heap abuse on fatties.

When Amanda cleverly discouraged a pair of Travis' flirtatious fans by alleging that the singer had barely recovered from a social disease, I was ready to walk. It seemed unlikely that "Georgia," a meaningless title inspired by a popular song, could rise above such a desperately coarse beginning. The prospect couldn't have been bleaker: "Back Roads" with a brother-and-sister gimmick.

The modulation to something more sociable and ingratiating colncides with the entrance of Mark Hamill in the roll of Conrad, a friendly, easygoing young patrolman who becomes infatuated with Amanda. The footloose siblings, en route to a recording date that Amanda hopes will revive Travis' battered career, are stranded in Conrad's town when Travis is picked up for vagrancy and aggravates the offense by assaulting a mean-tempered officer of the law, Don Stroud as a glowering brute named Seth. With the timely assistance of Conrad, Amanda arranges for her brother to work off his sentence tending bar at a local honky-tonk.

In this cordial environment, supervised by Arlen Dean Snyder as an affable saloonkeeper named Andy, the restless Childses and the grubby scenario begin to generate a belated but welcome human interest. Amanda, who has previously managed and goaded her undependable brother, emerges as a performer in her own right and lowers her hard-as-nails emotional guard against the calm, endearing efforts of Conrad to make himself indispensable. At the same time, a second romance blossoms between Travis and a local beauty, Melody, played by an adorable newcomer named Sunny Johnson.

This alliance also contains the seeds of screenwriter Bob Bonney's ultimate miscalculations: Melody has been going steady with Seth, who gets murder in his eye when she rejects him for Travis. The denouement is far more drastic than the slight material can support or the audience may be inclined to tolerate. After finding a playable groove, the filmmakers, succumb to a fresh attack of desperation, which leaves the story in poor condition at the fadeout.

Ideally, the producers would be encouraged to perserve the sturdy midsection of the movie and rebuild the beginning and ending in ways calculated to reinforce it. As a practical matter, the midsection will have to carry the show, obliging customers to put up with the surrounding wreckage.

The central episodes have the makings of a good little movie about small-town ambition and romance. When Maxwell and Bonney cease straining for vulgar yucks and hostile confrontations, they seem capable of sustaining a sweet-tempered, gratifying intimacy. The leading players have personality and charm to burn. They've even got four distinctive, irresistibly photogenic smiles. When the film concentrates on the ties that exist and develop between these four people, it's on solid, sentimental ground. You can believe in the family connection between McNichol and Quaid and in the romantic connections established between McNichol and Hamill on one hand and Quaid and Johnson on the other. The movie even uncovers a fresh subject: Amanda has a greater need to perform than her brother; indeed, her hunger for the limelight may be insatiable and ultimately irreconcilable with the interests of a nice guy like Conrad.

Kristy Mcnichol's performance provides a striking index to the shift in tone. Terrible at the outset, she rallies for formidable acting, resources as the picture itself improves. In the early going Maxwell seems to be asking for an impersonation of Tatum O'Neal at her lock-jawed, tight-lipped poutiest. The stubborn little girl as embodied by O'Neal in her juvenile heyday is the last cliche in the book McNichol needs to be wasted on. The most revealing aspect of "Little Darlings" was the contrast between her intensity and O'Neal's blandness. Obviously, McNichol was the juvenile star who also possessed the sensibility of a dedicated actress.

There's an extraordinary amount of expressive communication between Kristy McNichol and a camera lens. She seems wise beyond her years when it comes to projecting feelings through this powerfully suggestive medium. Ultimately, she projects something stronger than the filmmakers behind "Georgia" are prepared to acknowledge, and it gives the movie a slightly eerie note of pathos. Unless I miss my guess, McNichol doesn't buy the idea that Conrad could make Amanda happy. On the contrary, she seems to know that a fundamental conflict exists between the kind of ambition burning in Amanda and the kind of security offered by Conrad.