Didn't they pull codes? If it's running that badly I would expect to have some OBDII codes set. Do you have a McParts autostore nearby that will read OBDII codes for free? I would suggest starting there.

Plug wires are known to go bad on this engine, especially if the plug change interval has not been followed. I let mine go 7 years, but the car had just passed 50K miles. Surprisingly, changing the plugs improved the idle quality. It went from just a bit lumpy to perfect. No difference in driveabilty or mileage, though.

- JimY