Richard Stuart. Yo-yo syndrome. Assume that like most of us these days you're struggling to stick to the sensible diet. Still you can't seem to resist snacking on a hot dog smothered with onions, relish and mustard at a baseball game or a rock conert. How do you break your automatic habit pattern? That question was put to Dr. Richard B. Stuart, psychological director of the 600,000 members of Weight Watchers International. He was in Toronto recently to promote his new Book of the Month Club bestseller, Act Thin, Stay Thin (George J. McLeod, $11.50).