Gary Coleman is no longer cute. In "Playing With Fire," a two-hour NBC movie billed as the sitcomedian's "dramatic debut," airing Sunday night at 8 on Channel 4, he is truly mean.

He hurls a basketball at his helpless dog Fred. He grabs his little brother and shakes him like a rag doll. He shouts at his sensitive younger sister. And, as a troubled teen-ager named David Phillips, who turns to arson as solace for the disintegration of his parents' marriage, he burns down the house.

The show is an overlong epic of flaming inanity, featuring Cicely Tyson and Ron O'Neal as mom and dad and Yaphet Kotto as the friendly fire chief, with Gary huffing and puffing throughout. He begins modestly (cigarette lighter, coat rack), then graduates to a garbage can. Sometime later, it's bushes across the street, the neglectful O'Neal's child-support check used as kindling. Eventually, after a dust-up in the principal's office, it's an even bigger bunch of bushes at school before, if you still care at this point, the final conflagration.

The fires provide Coleman with his best moments as a serious dramatic actor. (The rest of his performance is all in the crease of his brow.) As he savors his blazing handiwork, his eyes give off a nasty glint, and his mouth contorts in a sinister grin.

This should serve as a warning to NBC executives, who are currently engaged in negotiations with the 17-year-old actor over his role in and payment for "Diff'rent Strokes," the network's longest-running prime-time series, which has been slipping in the ratings.

The other day Coleman threatened to walk off the set -- but better, perhaps, to light one match than curse Brandon Tartikoff.

For all the pyrotechnics in "Playing With Fire," producer/writer Lew Hunter's teleplay is a plodding bore, sleepwalking from fire to flare-up to foolishness -- "Chief, I called the fire department," Tyson says at one point. "I hardly would have done that if my son set the fire." (She wouldn't? ) -- toward the inevitable session with a family therapist.

Director Ivan Nagy has given the proceedings a generic designer gloss. Even the patients in a hospital burn unit look as if they came from the nearest modeling agency.

The climactic therapy session is arranged by Kotto, who resembles -- and might as well be -- Smokey the Bear. "I think you need help," he drones at Gary and his parents. "I think this whole family needs help."

Tyson, her hair teased to a high punk-de-doo, is overactive throughout, becoming a screaming meanie in the therapist's office after being told by the therapist, "We're not here to validate your marriage." If only the good doctor would just validate her parking ticket and get her out of there.

O'Neal, who at least conveys a sense of sanity, is the sole featured player who fails to annoy.

"Can't you stop hurting us? Stop hurting yourselves?" Coleman preaches at his parents. Later, in an epilogue after the happy ending, he perches in one of those canvas deck chairs favored by Hollywood stars and delivers a somber homily against teen-age arson. "Please, don't play with fire," he advises.

Despite his youth, he is in dire danger of becoming one of those rare performers who can blend smart-aleckiness with self-importance, on the order of Bob Hope and Frank Sinatra.