I have a ( I should say the wife has) a 03 Seville with about 95k on it. She has put about 5k since we bought it, One day it she complained of it being rough, so I though OK almost 100k I'll throw a set of PLUGS in it NO DIFFERENCE. Next I thought well I guess I should have plugged in my pos scan tool and look for CODES , there were NONE. Data stream showed extremely lean condition with LTF at +50 I think. Next I found the vacuum leak at the stupid rubber boot on the throttle body and replaced that and the intake gaskets since I was there.CAR STILL MISSES Intermittenly. Both long and short term are around 0 now. I finally got a code out of it PO300 whoopee. Reset and the wife drove it around and it came back with a PO305 so I pulled out the brand new plug and replaced it with another new one NO DIFFERENCE. I have looked for other vacuum leaks and havd found none using brake cleaner.
The car runs great if you nail it but still misses randomly.
Is it possible on these cars to unplug the idle air control valve and see an rpm change if it is working?

Thanks
Rich

 

Check your Coil Packs. Mine started out like that too and then one of the packs went out.