I hope they find nothing wrong, but something is causing that wandering and pulling to the left.
Regarding the pulling to the left, one other thought based on personal experience. We purchased a 1999 Silverado brand spanking new and it pulled to the left from the very beginning. I checked all the obvious things - tire air pressure, made sure the lug nuts were properly torqued, and I even rotated the tires and it still wandered. Took it into the dealership's service department and at the end of the day they said that the alignment was spot on. Well, on the drive home, it still pulled to the left.
I called the service department back, spoke to the service advisor who had grilled me the day before (to which I gladly answered his questions and let him know I had ordered a Factory Service Manual), and he asked me to bring it back in and meet with the shop foreman. I returned the following day, and the shop foreman and I went for a drive on a section of highway that had just been repaved, and it pulled to the left when I took my hands off the steering wheel. When we got back to the shop, he called over one of the technicians and instructed him to completely un-align the front end - to take it way out of spec, and then align it, and test drive it. Guess what. It no longer pulled to the left. It was perfect.
So, if the alignment shop doesn't find anything wrong, have them un-align the front end and then align it. What do you have to loose?