"Things Are Tough All Over" proves a prophetic bad-luck title for Cheech & Chong, who slump into the sporadically amusing doldrums for the first time in their brief but hilarious career as outrageous, disreputable movie clowns. Even the premise for this sputtering attempt at a picaresque farce seems to anticipate a vehicle prone to misfires and breakdowns. Hired to drive a limo from Chicago to Las Vegas, the improvident sidekicks are obliged to barter parts for gasoline. It's all too easy to equate the movie with their cannibalized automobile: each keeps running out of gas and ends up in pathetic condition.

Cheech & Chong certainly brought enough vulgar comic originality and gusto to their previous movies -- "Up in Smoke," "Cheech & Chong's Next Movie" and "Cheech & Chong's Nice Dreams" -- to be forgiven a lackluster outing. The first comedy team since Martin & Lewis to make an impact on the medium, they may also enjoy the indispensable luxury of a loyal audience. While this following tends to mystify many film executives and reviewers, perhaps too tasteful to relish the comedy of drug culture depravity as exaggerated by Cheech & Chong, its reliability has underwritten the team's improbable, successful transition from stand-up comedy and records to the movies.

The worrisome thing about their fourth picture, now at area theaters, is that it seems to find the stars growing uncertain of how to sustain existing comic identities while introducing fresh ones. Watching Woody Allen's "A Midsummer Night's Sex Comedy," one may fear that a favorite humorist is unable to develop new sources of comic inspiration after exhausting a vein that seemed distinctive and satisfying. In a similar respect, "Things Are Tough All Over" suggests that Cheech & Chong are searching for ways to finesse the inevitable aging of their original characters and the fading of the drug culture that inspired and nourished the idea of these oblivious, scroungy, amiable stooges. It remains to be seen if the iffy result of this role-shifting indecision will be remembered as a shaky but necessary transitional work or the prelude to a lamentable decline.

At one point in "Nice Dreams" Tommy Chong's character, the blithely stupefied doper known as "Man," as in "Hey, man," speculated that it might be lucrative to start the first rest home for aging hippies, since a number of acquaintances appeared to be ready for such a facility. "Things Are Tough All Over" may reflect a slightly premature anxiety on the part of Cheech & Chong that they'll wake up aging hippie comedians one alarming morning, but they're probably correct in assuming that only so much more can be expected of jokes based on the drug-induced misadventures of Man and "Cheech," the convivial Mexican-American reprobate created by Richard Marin, the more active and gifted physical comic of the team.

The weaning process they attempt in their new movie involves reducing the frequency of dope gags while distracting attention with other pretexts for slapstick and a second set of comic characters. Marin had played a dual role in "Next Movie," doubling as Cheech's irrepressibly jolly cousin "Red." Now Chong joins him in a dual role, as they portray shady, parvenu Arab businessmen, based in Chicago, who make the mistake of employing Man and Cheech in a succession of calamitous capacities, from car wash attendants to rock musicians to limo drivers.

The Arab caricatures have their funny aspects. Marin's affable, even-tempered Slyman, who's cultivating a head of transplanted hair that resembles a pin cushion dotted with porcupine quills, acts as the calming influence on Chong's Habib, a glowering Semitic hothead.

Unfortunately, the team doesn't necessarily double its effectiveness by appearing in dual roles. If anything, the switching from a familiar set of comic grotesques to an outlandish new set requires more getting used to than the skimpy continuity can support. Slyman and Habib set out in pursuit of their untrustworthy employes after learning that loot hidden in the car has disappeared, but the idea that Cheech & Chong end up chasing themselves generates no particular comic suspense or momentum, because each set of characters tends to get lost in the desert, where they revert to dialect and behaviorial shtick instead of making a little narrative headway.

In addition, the new set of characters merely postpones the problem facing Cheech & Chong if they hope to sustain a film career much longer: What should become of their basic ne'er-do-well characters as they get older? Can they even remain amusing as middle-aged vagrants, the cheerful remnants of a crumbling subculture? They already seem oddly displaced in "Things Are Tough," which relocates them from Los Angeles to wintry Chicago. It was probably cheaper to shoot on location in Chicago and Nevada, but the better routines don't travel too smoothly -- Cheech at the car wash criticizing the interior decoration of a pimp's gaudy chariot and then struggling to entertain a handful of impassive, uncomprehending Arab customers at the cavernous Club Mecca by leading into a fractured version of Ray Charles' "Tell Me What I Say" with parochial invocations like "How about a little trip down Memory Lane?" and "Remember cruising down Whittier Boulevard?"

Slyman and Habib would probably look more at home running their dubious "Two Guys from Mekka" franchises out of L.A. Wandering away from this home base in "Things Are Tough" seems to illustrate how much the team really needs it to keep free-floating, transient, hand-to-mouth characters securely afloat. The fond indulgence of casting their wives, Rikki Martin and Shelby Fiddis, as sex kittens -- a pair of French tootsies kept by Slyman and Habib but attracted to Cheech and Man--does nothing to enhance the comedy. On the contrary, it may have subtly inhibited the kind of boisterous sex slapstick that enlivened the earlier movies when the team got into ribald free-for-alls with actresses who were fellow comics.

Evelyn Guerrero turns up as Cheech's old girlfriend Donna, but she's deployed rather lamely, to set up one elaborate Polish sight gag about a Mexican-American tour group. Moreover, the usual abundance of striking young comic performers in secondary and bit roles has dwindled down to a single notable specialist, David Couwlier, as a kinko diner who tries to attract Chong's attention with eye and tongue semaphore during a promising but botched sequence in which Cheech and Man supposedly disguise themselves by going drag.

Perhaps Chong's decision to relinquish the directing duties he handled so astutely in "Next Movie" and "Nice Dreams" also put a hitch in the gitalong of "Things Are Tough." The first feature directed by Tom Avildsen, the team's former film editor and a younger cousin of John Avildsen, of fading "Rocky" fame, "Things Are Tough" has a lot more hitch than gitalong.

Presumably, Chong felt an increased performing burden because of the addition of Habib and the alterations in Man, who's become more of a talker and mixer as a consequence of losing his almost terminal preoccupation with exploring new frontiers of narcotic oblivion. The trade-off seems to weaken the new picture both behind and in front of the camera. Perhaps the surprising, rollicking momentum of the first three Cheech & Chong comedies couldn't be sustained, but who expected it to coast to a standstill in "Things Are Tough"? It won't be easy to restart their stalled comic engine.