The other possibility is a bad ignition switch or electrical to the ignition system, like a fuse holder or terminal causing a voltage drop to the box, at certain RPM could vibrate at that perfect frequeny. To rule this out just use a jumper for a quik run and see if the problem goes away.
I agree, sounds like an electrical problem. When you make the jumper wires, do power and ground to the batt. If it works fine than the fun starts

. Leave the power to the batt and put the ground back where you had it and try again (rules out a ground problem). You also have somekind of safty killswitch in there right? Check everything ! This will take some time to find, you need to bypass each component one at a time and try again until it works.
btw , if I remember right you had some problems with gauges not working correctly, right ?? might be related.