I just replaced my tach becuase the needle would jump up and down constantly, it would only jump back and forth in about 250 rpms over what the engine was really running at, but my new tach is all over the place it will sometimes jump up past 6000 rpm at idle activating my shift light and everything even at idle it will nto go below 2000rpm it just jumps back and forth between 2 and 6 thousand rpm......any idea's.....its on a sbc, with Mallory HEI........could my wirring be wrong and somehow be doing this? if so what is the bast way and places to wire your distributor? any idea's? thnx guys