Did someone mention checking the toe-in? Seems like the most logical way to start, assuming that there isn't excessive wear in any of the major steering components. As Wrenchbender mentioned, it's easy to check, easy to adjust to get a 1/8" toe in, and normally cures the problem. You didn't mention how much toe-in you had but I'd focus on that. If you've already got an 1/8" toe-in then I'd go with the heavier stuff.
