Just a thought, but check the coil wires where they plug into the coils. They are very loose normally, so they can easily come out enough to cause problems. I had 1 pop out completely when i hit a bump once and since have secured them with tie wraps. It could be that one is loose and at that perticular RPM for whatever reaon, electrical or vibration, it is intermittant. Takes all of a few seconds to put your hand up there and check so you may as well eliminate that as a possibility.
