So far, this summer's theater box offices have a disease -- and "Cobra" isn't the cure. Grosses are running well behind last year's totals, and though it's too early to make dire predictions, it's worth noting that things didn't get any better last weekend.

For the second week in a row the top three films were "Cobra," "Top Gun" and "Poltergeist II," but only "Top Gun" didn't show a big drop in its totals. It moved from third to second place in total gross and had the country's best per-screen average for the third weekend in a row.

Its competitors didn't fare so well. After opening with the year's biggest totals, "Cobra" fell by about 40 percent, with disappointing per-screen averages; it will make money, but its chances of becoming a "Rambo"-style blockbuster are fading fast. "Poltergeist II" also dropped by about 40 percent, though it's doing better on a screen-by-screen basis.

The Return of Prince

Two years ago, Warner Bros. held a sneak preview in San Diego for a low-budget musical drama they thought had mixed commercial prospects. But the overwhelming response that screening generated, both from the paying audience and from a few music critics who showed up, convinced the studio that Prince's "Purple Rain" might be a surprise hit -- and when they released it a couple of months later, that's what it became.

Last weekend, there was another San Diego sneak for another Prince movie, "Under the Cherry Moon," which opens nationally July 2. This time, of course, expectations are far higher, and some audience members reported that the screening clearly showed that Prince's star power is undiminished: The young girls in attendance screamed whenever their idol appeared on the screen, and did so with special vigor when Prince teased and flirted with the camera.

But beyond that, the screening was less of a success: "It's moderately likeable, but that's about it," said one moviegoer who attended the "Purple Rain" preview and remembers that film having a far greater visceral impact.

The new film -- which Prince directed -- is also far different in tone. Set in Nice, it's a relentlessly light comedy with Prince as a gigolo out to marry a rich heiress for her money. She falls for him; her parents, naturally, object; and finally, after relentlessly light, relatively predictable romantic comedy proceedings, there's a surprise ending.

At any rate, its prospects seem far more questionable than "Purple Rain's" were, and some who previewed it expect critics to disapprove mightily, both of the film and of Prince's apparent new-found taste for high society. "He seems awfully fascinated by making big bucks and living the glamorous life," griped one viewer after the screening. "I mean, the whole movie looked like the black and white ad pages in Interview or Vanity Fair . . . "

Two Lost in Space

"Under the Cherry Moon" wasn't the only film to pick up some negative word-of-mouth lately. Take two of the major studio films that open tomorrow: Outside of 20th Century Fox, few folks have worked up much enthusiasm for "Spacecamp," while Tobe Hooper's remake of the 1953 sci-fi adventure "Invaders From Mars" has been unanimously panned.

To be fair, though, "Invaders" has two lines that drew laughs at its previews. At one point, a Marine commander leads his men into battle and reassures young Hunter Carson with a gruff, "Don't worry: Marines have no qualms about killing Martians." And early in the film, when Carson first realizes that something strange has gone on at the top of the hill behind his house, the police arrive to investigate. The police chief is played by Jimmy Hunt, who portrayed the frightened youngster in the original "Invaders" -- and as he walks up the hill to take a look, the chief mutters, "I haven't been up this hill since I was a kid."