I would: change all spark plugs, change all ignition wires, change all the coil packs and the ignition module. I would then have the suspension examined by well known shop and make any changes that they recommend. The Grand Prix is not a sport handling car; it's a road car that looks and feels great going fast in a straight line. My gp was getting 29/22 mpg hwy/city and it averaged about 25 mpg with about 80% freeway driving. I absolutely love my car even now when there is a miss on my #5 cylinder and my mpgs have tanked a bit (mostly due to me not putting it on the freeway). It now has 201K miles and I know a guy that has 2 that have 260K miles or better and he still drives the freeway with them....