I have noticed sometimes that my tach needle generally works smooth and steady but periodically it will jump up for a moment like it has a seizure or the throttle was barely blipped, and then it is fine again. This has been observed both at idle, like at a stop light, and while accelerating especially at higher RPM's. I can't predict it but it does it randomly during pretty much any drive. I don't notice any change in engine performance during this split-second jump.
I assume it means the signal is getting interference somehow but it's so brief that the needle doesn't jump more than say 20 RPM - where should I be looking? I did replace the impulse coil with a NOS coil a couple months ago when I had my distributor out.
Ignition coil is an Accel Super Stock coil and perhaps 20 years old. I have a new Bosch rotor, DMCMW repro cap, DMCMW ignition wires and coil wire, and NGK plugs, all installed this year and properly gapped.
Here's a video showing the tach jump at about 5 second in.
https://youtu.be/hBG6znVDcBU
What is the cause?