Somewhere in Time is the most boring movie since "The Blue Lagoon," which is a mighty dubious distinction. Christopher Reeve plays a playwright who travels back in time. How?By closing his eyes and wishing real hard. There are comical possibilities in the predicament of a modern man stranded in the past without so much as a suitcase or a change of socks, but the movie ignores these in favor of a tedious love story. Everything plods along without a shred of energy, to a Rachmaninoff-derived score that sounds like Mantovani aroused in the middle of the night and not fully awake yet. For director Jeannot Szwarc, this is quite a comedown, even from the depths of "Jaws II." As for Mr. Reeve, the sooner her changes back to his Superman suit, the better for us all.