1991 shakes and sputters at highway speeds
1991. all stock. 51,000 miles. auto trans. L98. while driving at highway speeds, mostly around 65mph, it will shake and sputter and have little throttle response. no waring and nothing else happens, rpm's don't jump up or down, no warning lights on the dash. when I let off the gas everything smooths out fine. The cars idles fine and runs great at lower speeds. No problems under 55mph. I've owned the car for months and it just recently started doing this. It's driven weekly and I always use 93 octane.
Any ideas what might cause this? Thanks!