My '98 Deville has 90,000 miles on it and a few weeks ago it started to run rough. It launched badly and stumbled and started getting very poor mileage. I didn't go whole hog on it but changed out the 8 plugs and fuel filter for about $150. That worked well until we got this real cold snap of -15 to -20 and it just started to stumble and the idle was bouncing between 550 and 800 rpm, it would backfire from time to time as well.The car would not code.

What I found was that the coil pack with #7 cylinder had the terminal completely rusted and for good measure a new set of wires went on. This cost $90 for the GM coil pack and $240 for GM wires. The good news is that the idle is smooth as silk and my power and acceleartion are back to what they sould be...awesome. My 17 year old son left the driveway with the front tires pealing for about 50 feet. When he returned he mentioned that the Traction Control was ON!