Tonight the steadfastly praiseworthy PBS drama series "American Playhouse" exercises its right to fail with "The Great American Fourth of July and Other Disasters," a big burp of macho cuteness by the overrated radio humorist Jean Shepherd.

The one-hour comedy, at 9, or shortly thereafter, on Channel 26, suffers from a severe case of underdramatization. It would be a far more tolerable indulgence if Shepherd would only shut up, but failing the basic abilities of a playwright, he insists on narrating the entire story. He becomes an intrusion upon an intrusion and proves tremendously unfunny.

Ostensibly a blowzy reminiscence of a midwestern summer in the tacky '50s, "Fourth of July" stumbles aimlessly from one self-conscious facetiousness to another. It isn't helped by Shepherd's introduction, which finds him driving a gray Rolls-Royce down his beloved Interstate 95 and waxing clumsily and redundantly ecstatic over this or that garish hunk of pop-cult: "Wouldja look at that? Wouldja look at that?" and "I love it, I love it" and "This is great." To throw one of his own witticisms right back in his face, "Oh, ugh."

Among those trapped in the mess are James Broderick as a beer-swilling, firework-popping, middle-class husband and Matt Dillon (of "My Bodyguard") as his son, a young Shepherd, who does salvage an instant or two out of a twist on the old blind-date gag. He'd heard the girl was pudgy and wore braces, but when she appears, she has ugly-ducklinged into a gawjuss blond. And then suddenly he learns the mortification of being the encumbrance rather than the encumbered.

Director Dick Bartlett seems intent on amplifying every annoying thing about Shepherd's cryptic, smart-aleck style with a wrongheaded whimsical touch of his own. A musical score of sorts was cribbed from George Gershwin ("An American in Paris"), Nino Rota ("8 1/2") and other unwitting sources. Shepherd tries to be cheeky, but he remains precious and innocuous. Is he the poor man's Mort Sahl, or the utterly destitute man's Studs Terkel, or what?