When the motor is cold, it starts fine, idles fine and has plenty of power taking off. While running comfortably at 5000 rpm and 45 mph, after several minutes the motor clearly starts "jumping" (i.e random, quick bursts off power every couple of seconds). Sometimes when this happens I can hear definite "knocking" in the motor. After a minute or two of "jumping", the motor then will only run at 4000 rpms and 35 - 40 mph. While running at this reduced speed/power, it rarely, if ever, "jumps". I've checked plugs/wires etc. I ran some fuel injector cleaner through the motor and that helped for a couple of short trips, then the motor starting "jumping" again. It almost always runs great while cold. Does this sound like a dirty carb issue? Could it be a coil randomly misfiring? Other suggestions?