knock sensor retarding timing

My '96 Pontiac Grand Prix with 147K on 3.1 V6's knock sensor has taken to retard the timing to 0 and less. It only does it after it has been driven for about 15 minutes or so. Then, as you excellerate up to say 50, at about 2,000 rpm it quits excellerating, untill it finally downshifts, and then it runs great. Once you level off, if you try to accelerate, nothing happens untill it downshifts again. Once it gets over 3,000 rpm, it'll run fine. But who can cruise at 100?