2 weeks ago I changed my head gaskets, and within that time frame I have had to replace the right head, starter, ICM. I have figured out all issues but these. I have replace all wires, and plugs and still is crappy at the lower speeds and idle. I have switched coils and even bought new ones. I do know before the gasket change I was getting P0420 code. By the way its a 2003 alero 3.4l 6 cylinder with only 134,000 miles on it. I am done trying to guess at what it may be. If any one may have a clue, please un tie my mind because I am perplexed now.