In between movies, a batch of comedians, including Chevy Chase, Buck Henry, Laraine Newman and Robin Williams ("The most human being you'll ever meet," according to ads for his title role in the upcoming "The World According to Garp"), found time to perform sketches and monologues at a Hollywood benefit for the National Committee for an Effective Congress, a group that says it's trying to counter "the enormous amount of money raised by the radical right." But it was some backstage shenanigans that have many of those involved angrily muttering under their breath; comedians are known for being difficult, and some of those difficulties threw a monkey wrench into the organization's fund-raising plans. With much of the press reluctant to publicize the show for fear of appearing to support the cause, ticket sales were extremely slow; a planned spot on "Entertainment Tonight" had the potential to change that, until Chase decided that he didn't much like that program. According to reports from the scene, Chase stood just off camera, and while an "Entertainment Tonight" reporter tried to conduct interviews, he kept up a running commentary of scurrilous, sometimes downright obscene lines. Exit the angry "Entertainment Tonight" crew, their interviews unfinished and their story scrapped. To complicate things, several insiders confirm that a planned sale to cable TV--which was needed to put the event comfortably into the black--was nearly canceled when Robin Williams decided he didn't want to be on cable. Williams later relented, the cable sale was finally made, and an amazed organization--which the previous day had tried to paper the house with free tickets--watched the event turn into an 11th-hour sellout. On stage, Williams even offered a thumbnail summary of his own experience in films: "I used to have a career, you know. Now 'Popeye' is playing on double bills with 'Heaven's Gate.' "