I just installed the auto meter 5" phantom tachometer and its causing me some trouble. At and idle, the tach jumps from 500 to 1000 rpm, but when I hit the the throttle, the rpms reading on the tach are normal and stable.
So I tried a few different 12 volt sources and that didnt change anything, then when I disconnected the ground, the tach read normal without jumping at all so I tried leaving it like that but then the light and the shift light dont work. I have and HEI distributor.

Please Help!!