"Trading Places" is a comedy of unavoidable fits and starts.

This diverting and generally affable costarring pretext for Dan Aykroyd and Eddie Murphy, opening today at area theaters, solicits comparisons in its press material to such Preston Sturges' comedies as "Easy Living" and "Christmas in July." No doubt Sturges was an inspiration, but "Trading Places" is far too spotty to profit from the association. If anything, screenwriters Timothy Harris and Herschel Weingrod fail to contrive an acceptably updated setting for the comedy of social contrasts that flourished in Hollywood throughout the '30s.

The plot is generated by the capricious maliciousness of a pair of elderly financial opportunists, played by Ralph Bellamy and Don Ameche, both in such accomplished form that you may resent the filmmakers for heaping an excess of moralistic abuse on their troublemaking characters.

Bellamy and Ameche portray Randolph and Mortimer Duke, the founders and still actively devious directors of a commodities brokerage company. An ongoing difference of opinion about the relative importance of environment and heredity in character formation leads the tyrannical old boys to settle a wager by conspiring to arrange an experiment in overnight social transformation. They'll exchange the privileged, overgrown preppie type--Aykroyd as Louis Winthorpe III, who manages their firm and is engaged to their grandniece--with a black petty criminal--Murphy as a young street hustler named Billy Ray Valentine, who specializes in a literally lowdown scam, panhandling while masquerading as a legless war veteran.

The sudden change of fortunes appears to reinforce the position of Randolph Duke. Socially elevated, Billy Ray soon displays a natural, er, flair for commodities speculation and promptly fills Louis' shoes. Socially disgraced and degraded--a process that must be taken with choking grains of salt, since the episodes strain credulity even in a farcical context--Louis veers to the brink of genuine criminality and then suicide.

Rescued by Billy Ray, whose basic decency and freshly minted self-respect cause him to resent the Duke conspiracy as soon as he wises to it, Louis joins with his reluctant replacement in a scheme designed to turn the tables by shorting the market. Billy Ray and Louis are enthusiastically aided by two confederates--Denholm Elliott as a conscience-striken valet who felt compelled to play along with the hoax initially, and Jamie Lee Curtis, a cheerful comic discovery in the dismally stale, predictable role of a hard-shelled hooker concealing a heart of gold.

The material demands a considerable amount of fleetness and finesse if a director hopes to outmaneuver the basic spuriousness of it all. Landis seems to have a genuine affinity for comic performers and situations, but his timing and judgment don't seem as deft as they were in his breakthrough projects like "Kentucky Fried Movie" and "National Lampoon's Animal House."

Perhaps he's inclined to let the slack accumulate when switching from intimate to overelaborate comic byplay. For example, the movie's first sputtering signs develop during an excessively complicated sequence at Louis' club when he's framed for theft by an agent of the Dukes. Something is out of whack, and I think it's a question of the buildup being too grandiose, especially from the pictorial standpoint, for what turns out to be a rather lamely inserted and marginally believable deception.

In a similar respect, there's an involved reciprocal sequence in which Aykroyd, Murphy, Curtis and Elliott enter a train compartment in facetious disguises to carry out a deception aimed at the same Duke flunky, played by Paul Gleason. Instead of accumulating humorous momentum, this parade of masqueraders peaks with Murphy's opening routine as a happy goof from the Cameroons and then bogs down in a confined setting, posing no serious challenge to the stateroom sequence in "A Night at the Opera."

There are letdowns of other kinds. Jim Belushi makes a brashly funny appearance as a merrymaker in a gorilla costume at the start of the train sequence, but he's never coordinated with the subsequent actions--which ultimately involve a caged gorilla--as wittily or extensively as you hope. I doubt if anyone outside the brokerage business will be able to comprehend how Louis and Billy Ray contrive to sting the Dukes on the floor of the commodities exchange, and the brokers will recognize that it hinges on a deceit as farfetched as Matthew Broderick's interfacing with a Defense Department computer in "WarGames." In this case sheer uproar and rooting interest combine to carry the climactic scene--you aren't sure what the heroes are up to, but they are the heroes.

Billy Ray doesn't provide Eddie Murphy with a significantly enhanced showcase, but it does allow him to confirm the exhilarating comic authority and extraordinary camera presence that proved an immediate movie sensation in "48 Hrs." If anyone was tempted to regard that performance as a fluke, "Trading Places" serves as immediate corroboration of his talent.

Aykroyd has his most appealing and satisfying movie role to date in Louis. Sharing the comic load certainly does more for him than struggling to sustain it solo in "Dr. Detroit." Louis incorporates his smartypants side in an ingenious way--when the character can no longer afford his original pomposity, Aykroyd can emerge a more likable comic lead. He's better than the material warrants. So are Curtis as the helpful hooker and Kristin Holby as the uptight fiance'. Despite the triteness of the role, Curtis brings such unfailing, infectious good humor to this project that one shares in her evident feeling of liberation from the horror genre.