My engine is original and never been opened. I've had similar idle problems through the years. Once it was the IAC. Got a new GM IAC problem solved. Another time, I had 3 loose intake bolts near the egr ports. torqued them down greatly improved idle. Also check EGR valve. If original its probably got a blown diaphragm and leaking. MAF can also cause strange Idle problems. Ive also seen bad TPS sensors and misadjusted TPS sensors cause poor idle. Often the SES light will store codes that you can easily readout that can help you narrow it down. Last, chase down all the vacuum lines. Original lines get porous, soft/hard and leak which causes all kinds of problems. Make sure if you have a K&N you have the AT sensor plugged in and installed in the duct before the MAF.