i have a 90 sc which just got new fel-pro head gaskets. upon starting the car revs up then drops to a idle perfectly. it holds for about 20 seconds then the rpms start to fall off and the car starts shaking and emits a terrible rattling. i know the car has to learn to idle again but the rattling noise has me concerned. it only makes this noise as it is dying. i've revved the car to red line with no rattling at all. also if i rev the car up a couple times after starting it, it will idle fine. any suggestions?