So I've got my head re-worked recently, and since then, the car runs like crap (bogs down, backfires) when the car gets up to temp. About 2 or 3 seconds before it starts messing up, the tach jumps around like crazy from 1k to 9k. I've searched for a bad ground wire, but can't find one. I really don't think it's the timing, because the car runs great for the first 10-15 minutes, until the engine gets up to normal temp. The car never overheats, even when I'm letting it idle for about 30 minutes to look for problems. Anyone have any idea what the problem could be from?