""CLIFFHANGER," the new high-altitude thriller starring Sylvester Stallone, wastes no time in establishing its first priority, that of sending its audience into a cold sweat. This effect is achieved spectacularly well with a long, death-defying credit sequence set somewhere near the ozone layer. Mr. Stallone, playing a pumped-up Rocky Mountain rescue worker with an incongruously Presidential haircut, does his best to save a young woman who hangs precariously from a rope across a terrifyingly deep abyss. You will be in no hurry to mountain-climb after watching this scene. At such moments, and "Cliffhanger" has many of them, thoughts about life insurance come to mind. Was Sylvester Stallone really filmed atop the dizzyingly high, needle-shaped peak that figures prominently in the film's opening? Did he really scale the kinds of cliffs and mountainsides that are seen here? Did he hang in midair from a fraying rope, cross great chasms via rickety bridges or battle over a helicopter about to plunge into oblivion? Obviously, a great deal of credit goes to stunt doubles and optical tricks, but the illusion is still astonishingly effective. "Cliffhanger" really seems to be set somewhere up in the sky."