Tach has irregular readings

[Marko911/944]

My 83' 911sc cab has a lopping idle and a crazy inconsistent tach reading. When I get it on the road it has trouble revving to mid range. it starts missing, and only runs smooth at idle. Any ideas? I once hooked up a quick start battery charger to the wrong terminals, did I jack up anything specific?

[j911brick]

Just based on what you have mentioned, sounds like the two problems are related and probably and ignition issue.