I've got a GT conv. 4.6 from the 97/98 era, and the customer is complaining of a rough idle. It does seem to have a slight miss. Intake was recently replaced because it was overheated. Also, the vehicle received a complete tune up not too long ago. Following checks were done and it seems to be passing everything. Sprayed the intake down and found no leaks. TPS is good. IAC checks good-disconnected and car engine started to stall. Vac. test shows -20 and steady which is right on. All plugs are firing and all the plugs look coffee and cream. Sprayed out throttle body with cleaner. Engine has no codes and passes both self tests. Fuel pressure is about 32-35 psi which according to my Matco book is right on. From the backside, the MAF looks clean but I didnt actually look at the thin wire. Its an automatic with 92k miles. The customer takes good care of the vehicle. The reason the intake was replaced was because the engine was overheated and the intake cracked or warped. The vacuum test would have shown a bad valve, or incorrect timing, vacuum leak or plugged exhaust. At one point I did smell the rotten egg smell but thats just condensation in the exhaust. The miss seems to be present all through the RPM range, but the roughness is at idle, and more present with a load.