Things haven't been terribly encouraging at the box office lately: None of the movies that have stayed near the top of the box office charts ("The Money Pit," for instance) has been setting any records; meanwhile, other films have opened with enough muscle to beat the competition but didn't have staying power ("Murphy's Law," for one, seems a likely candidate for a quick fade).

But now it's May, which means the Big Movies are almost upon us. Summer in the movie business seems to come a bit earlier every year: This year, it should arrive in mid-May with the release of "Top Gun," "Cobra" and "Poltergeist II." Serious studio hopes are riding on all three films, but in a way the fate of the last of them may be the most telling of all because, as often happens in this town, there's a definite theme emerging in 1986's summer movies.

Late last summer, the trend was to teen-age science comedies, and all of them died. (In case you forgot, they were "Weird Science," "Real Genius" and "My Science Project.") Before that, there was a string of commercially disappointing westerns. ("Silverado," "Pale Rider" and "Rustler's Rhapsody." Remember?) And this summer, "Poltergeist II" is the first of a lot of movies about things that go bump in the night -- or, to be more precise, things that go rebump in the night, because more than half the summer's horror films are sequels.

As The Hollywood Reporter noted in a recent roundup, there's a chiller due every few weeks. "Poltergeist" will have a head start of six weeks or so; then "Psycho III" comes along in early July, followed two weeks later by the "Alien" sequel, "Aliens." Then there's Stephen King's directorial debut, "Maximum Overdrive"; Grace Jones in the horror comedy "Vamp"; a remake of "The Fly" from David Cronenberg ("Scanners" and "Videodrome"); "Friday the 13th, Part VI" (opening, for what must be good reasons, on Wednesday the 13th of August); "Deadly Friend"; and last but almost certainly not least, "Texas Chainsaw Massacre II." And then summer will be over, the kids will be back in school and the serious movies will come out. And they say the seasons don't change in Los Angeles. Ross Hunter's Missing Frames

Producer Ross Hunter recently told Daily Variety that while he was checking the quality of the transfer that took his film "Airport" to videocassette, he noticed that 11 of the movie's frames were missing. "I know every frame of my pictures," he explained. Movies are shot at 24 frames per second; each frame is then shown twice for a projection speed of 48 frames per second. "Airport" is 137 minutes long. That means it has 197,280 different frames. And Hunter noticed that 11 of them were missing. Somebody oughtta put that guy in pictures. Ringwald's Development Deal

Molly Ringwald is an actress, a teen-ager and now, a fledgling mini-mogul. Ringwald, 18, has signed a development deal with United Artists to develop feature films for the studio, both films starring Ringwald and projects for others . . . Meanwhile, Paramount has chosen Kevin Costner ("Silverado") to fill Robert Stack's shoes playing Eliot Ness in the big-screen version of "The Untouchables." End Notes

His last movie was awfully stylish and his new one's written by an ex-television writer and set in Miami, but don't look for pastels, expensive clothes or pregnant pauses in Dan Bacaner's "Scared Stiff." The executive producer of "Blood Simple," Bacaner is in Miami working on the low-budget film, which was written by former "Hill Street Blues" writer-editor Mark Frost . . . Also in Miami, principal photography began three days ago on Making Mr. Right," a romantic comedy from director Susan Seidelman, who won an underground reputation with "Smithereens" but got far more attention for "Desperately Seeking Susan."