These cars are VERY sensitive to differences in toe. Get it set dead-on (or as close as possible) with NO play between the left or right settings.
If you've racked up a lot of miles or have beat on the car in terms of handling, the A-arm bushings could be on their way out, contributing to your issues...
Check them for wear and keep taking it back to the shop until they do the job right, or find another shop. I seriously doubt its the tires.
