I was trying to figure out which heatshield was causing a vibration/rattle as the car decellerated, so i slowly reved the car (out of gear) and at 2000rpm i heard the vibration and figured it was the cat heat shield, but then as i reved a bit more and held my foot steady on the gas, the engine revs dropped to 1000rpm and then back up to 2k, than back down, then up and so on. I revved a little higher (3krpm) and it held fine. So i drove in first at 2krpm and it drove fine and the rpms remained at 2k. so i stopped again to see if it was still happening at 2k and it was!. I'm not getting any codes and figured that the cold start valve in the throttle body might be gummed up again, but i'm not sure. the engine was up to full temp when this was happening. i'll have to wait a while till the engine is cold to see if it does it them...any ideas on whats going on?