This is the most knock-sensitive region of the RPM range - have you checked your timing lately? Should be 12deg +/-2deg BTDC. And what grade of gas do you run? I have my timing slightly advanced, and I need to run 89 to avoid knock-sensor retardation (which feels like hesitation or missing as the timing is pulled back in large chunks.) Try a tank of the super 94 stuff and see if the problem continues. If it does, check your knock sensor to make sure it's working - the procedure is in Haynes, put key in position II with engine off and backprobe knock sensor looking for 2.5-5.0V, I think. I had the ignition control unit go bad on my '83 (wouldn't provide the proper bias voltage for the knock sensor), so the ignition was retarded at random all the time.