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It's back! "Diagnosis Murder," the Dick Van Dyke whodunit, returns for its eighth season on CBS at a new time, 9 p.m. on Channel 5. This is the show of which CBS-TV president Les Moonves recently said, "You can't kill this thing." Heaven knows he has tried, putting it on TV's toughest night four years ago, only to see it turn into the Van Dyke Full Employment Program. (Besides Dick, son Barry is a cast regular and brother Jerry and grandson Tyler make guest appearances.) Now it's in an even bloodier time period, opposite "ER." Though the ratings for "Diagnosis Murder" have slipped, its audience of older viewers remains faithful (remember Uncle Junior falling asleep to it on "The Sopranos"?). And Van Dyke has made the show more than just "Murder, She Wrote" in scrubs: Recent episodes preached against HMOs and the poor treatment of Alzheimer's patients. Another was staged as a Western. Tonight: The guy hired to remodel Van Dyke's office winds up stuffed into a roll of carpet, and the hunt for the murderer leads to Egypt. Later this season: Ideal viewer Sonja Christopher, the 62-year-old woman voted off on the first week of "Survivor," is a guest. @ART CAPTION:Dick 'Diagnosis' Van Dyke @ART:Photo (color) >>>