did you just put a new tach in? the reason i ask is that if you put an aftermarket tach in, autometer for sure, has three(if i remember right) small wires in the back that you have to cut for the number of cylinders you have. this happened to me about eight years ago, it turned out the tach thought i had a six cylinder, after i cut to of the three wires it was fine.

dont know if this helps or is relevant to you, just takin a stab at it