Fiery talk-show host Morton Downey Jr. may be trying to buy his low-rated TV show in a last-minute bid to save it from cancellation, but a spokesman for the companies that produce and distribute it said Monday he doesn't know exactly what Downey plans. "We don't know any details of this supposed deal," said John Murphy. He referred to reports quoting Downey as saying he intends to start negotiations to buy his show from Quantum Media Inc., which produces it, and MCA Television, which syndicates it. Downey told New York papers that he would change the show from a daily to a weekly series, adding, "My position is and always has been that five times a week is overexposure for this type of persona." Downey's boisterous, much-criticized, confrontational-style show started as a local program in October, 1987, at MCA-owned WWOR-TV in Secaucus, N.J., where "The Morton Downey Jr. Show" now is taped. Amid a flurry of publicity, the series went into national syndication a year ago. At its peak, it aired on 85 stations, Murphy said. However, many stations, including KABC-TV in Los Angeles, have axed it in recent weeks. Only 24 have renewed it, leading to speculation tha Quantum and MCA may cancel it. Murphy said executives at those companies "still haven't made that decision yet."