Some good posts here already, but a note on tires. If it's pulling to the left (w/good alignment) check the RIGHT tire, and vise versa.
I've done the string alignment for years and have never had a problem with it. The more complicated machines get, the more things can get outa whack and nobody knows it's happening and if the did wouldn't have a clue what to do to correct the prob.
My guess is you have belt shift on the right front tire.