Change your plugs, plug wires, air filter, fuel filter (might as well hit it), check your tires for proper inflation and then see how things react. Always start with the cheap stuff if you haven't done any maintenance of the sort in a while and if you don't have a specific idea of what your problem is.
I can tell you right now that it ISN'T your rims, unless they weigh in about 50lbs. a piece and you're running 6-ply trailer tires on them...
The lack of a CEL has got me questioning whether it could be your O2 sensors, but have your local shop check them just in case (stranger things have happened on Contours). Also have them check your ignition timing as well.
It could also be that your cat is getting clogged up, too. There are just too many variables for one swipe, so you're going to have to start eliminating some of the cheap and easy ones first.
