JASON ROBARDS' STILLAR performances as a supporting actor won him several academy awards (in 1976 as Ben Bradlce in All the President's Men and the following year for Julia). But when he made the jump to starring roles, his films could not cut it.

The same is true for funnyman John Candy. As the perverted older brother in Splash, he added life and silly comic wit to what was otherwise a fun, but not necessarily funny story. In Vacation, his brief appearance provided a welcome relief to Chevy Chase's not so funny slapstick.

But Candy's performance in his first feature film is only slightly funnier than Jason Robards' starring performance in The Day After. Actually, the script of Summer Rental has more to do with this movie's quality than Candy's performance does, and one feels sorry for the rotund actor who tries admirably, but can't save this sagging film.

Summer Rental seems like a film Chevy Chase would make. It comes complete with Jack Chester (Candy) as the anxiety ridden father, and his caricature family, all of whom are off to Florida for a month-long stay in a summer beach house. One enters the theater hoping that Candy can pull the stunt off better than Chase ever did, but leaves with the feeling that this movie would have been better with Candy cast as one of the sidekicks.

The Chester family is perhaps the world's most picked on family. After waiting for several hours in line at a posh restaurant, the last five lobsters are taken from right under then noses by the restaurant's most special customer (who also had the decency to cut the line right in front of the Chester clan.)

The Chesters also find they must move from the house which they think is theirs (beautifully decorated, with its own private beach, several balconies and cable television) to the house which is actually theirs (a veritable pit, located right on the access-way to the public beach and with construction going on all the time next door.)

Once they are all settled, the movie loses steam. It is reduced to a series of Jack's clashes with the lobster restaurant's very special customer who, at least in his own mind, is somewhat of a big deal around Citrus Grove, Florida.

The ultimate contest comes when Jack challenges the seafaring snob to a sailing duel. They wager two weeks' rent (or about $1000) on the outcome of the race.

Of course Jack knows little about sailing before arriving in Florida (he crashed into his landlord's boat the first day out), but through a friendship he develops with an of pirate with a hook arm, he learns the ways of the seas.

The other disappointment about the movie is that it ends immediately after the end of the sailboat regatta. It should be obvious who wins the race. And this makes it all the more imperative to leave the audience with something else in their minds besides a predictable ending.

THE MOVIE DOES HAVE its few funny points, such as Jack's sunburn. After Jack's first day out on the beach, it's hard to tell who is redder, him, or the lobsters he is about to eat. He is so burned that even driving is painful: "I'll be all right, as long as we don't have to make too many turns," he comments.

A scene where Jack is getting his sea legs from the pirate Scully takes them out to the deep sea, where Jack asks his mentor if he knows any seashanties. Of course, the venerable seaman responds, he knows "a many sea ditties." He proceeds to sing a "seashanty" that turns out to be the theme from The Love Boat sung in an limey sailor's accent.

Aside from these bits, though, there's not much reason to see Summer Rental. Wait for Candy to do another supporting role (he teams up with Tom Hanks again next week in a film called Volunteers), or at least until he gets the lead in a movie with more of a plot and supporting cast than this one.