Originally Posted by Ra005e

Just to be clear, when the rpms jump can you hear the engine doing that? Do you feel a surge, etc. if it's in gear/drive?

I'm thinking it might just be the tach acting up :hsdunno: I mean, if you rpms were actually reading zero the engine would stall...

Anyway, if it is in fact the tach acting on it's own without relation to what the engine's doing, I'd first check all your grounds to make sure they're clean & tight (I sound like a broken record).