if its fine on some roads and bad on others it may be the road and not your car. My car will go straight as an arrow as long as the road is perfectly flat, but there is one road on my way to wal-mart that is crowned... now with my winter tires on (non-low pro) its no problem but with my summer tires (low pro) my car is a friggin bitch to drive on that particular road. she pulls all over the place. And to the eye you can't even really tell that there is a crown in the road, but with those tires on you can feel it...
Anyway make sure you are on a good road before you start judging your alignment by it.