running bad after tach install?????

i installed a tach the other day and now my rpms jump up and down at idle..i wired it to the neg side of the coil and the power the the old radio power the illumination to the headlights and used the ground from the radio..cleaned it up ofcourse..i used all heat shrink terminals and the right size wiring and double checked everything..it jumps about 5-600 rpms and comes close to stalling.. anyone have any ides on what might cause this or a way to troubleshoot it??? thanx in advance