1984 Pontiac Fiero Electrical load messes with my RPM

Whenever I put electrical load on my car while in gear my tac goes down. With the turn signal it goes up and down :). It idles a little rough in gear anyways. I have new Alt and Batt. I cleaned my batt connection (just the batt end of cables). I tested, hopefully, all my grounds. Oh, also, on day after driving I turned it off and 2 min later it lost *all* electrical except starter solinoid (it clicked but that was all). I crossed checked the batt and system in another car and it worked fine. So I reconnected batt and it was fine. After which I cleaned terminals. I suspect that it is related to the rough idle in gear, but not sure. Any ideas?