Tach drift/wander

Has anyone ever had this problem? I've heard of it on the newer stangs but it seems to be a computer problem. Not sure what it could be on mine. The idle speed wanders when the vehicle is stationary and in gear, after the idle kicks down to regular curb idle. Goes from about 900 to 400 rpm (in gear) with rapid transitions. I've ALSO been having problems on frosty mornings with the fast idle speed not keeping pace (too slow) even though I've adjusted the screw to where its supposed to be (on a warm afternoon) AND it sounds really rough (chop-c-chop-chop-chop-c-chop, etc) after a couple minutes of warm up on those cold frosty mornings.

I'm not sure if these are all related or individual problems. Doesn't sound like the finely ticking beast I'd like it to be. I've recently changed the carb, ignition wires and coil, and given it a tuneup. My thoughts are turning toward the ignition "box" (thingamiggiger) and fuel pump but to be honest I'm clueless