Hello Matt,
Where about do you live? I'd suggest you get someone with a tuner to look at your sensors to see what readings they are giving before replacing anything. I went through $1000 in parts on my GTP once to find out it was just my MAF sensor that went and didn't even throw a code. If it did, it would have been only $150
I know SBC has DHP tuner in the east end of durham, or a someone who really knows his stuff is Will (Overkill) on the grand Prix forums. He could probably give your car a complete tuning and tell you why your engine isn't happy.
http://www.getoverkilled.ca/3800.html