Markarino

Hey guys, I've came across the strangest tachometer problems on my pickup. The tach just doesn't read properly, its usually off by 400 RPMS and this is only during idle. While driving it appears to be working properly judging by the sound the motor is making. But once I come to a complete stop the tach will stop at 800 then drop to below 500. The motor will stay idling at 800 but the tach just appears to have a mind of its own. There are even some days that it works perfectly fine. I've already changed the tach itself and still no difference so i'm going with a wiring issue. I'm just not sure what wires I should check, I know its something with the igniter but I'd like a better lead if anybody has one. Thanks guys.