[pcv]

I sympathise with you on this one. Call the dealership up and talk to the Service Manager and explain the situation to him. If you are not far from the dealership, they will send some one over to give it a quick check. Because, once you tow the vehicle, and go over a bump, whatever is causing your problem, resets it self and by the time it gets to the dealership, it starts and it makes real difficult (especially in these situations) to pin point and fix the problem. Anyway, talk to them. Second time around they will give it more attention.
Good Luck.