The first time this bucking happened at high speeds to me, was last summer after purchasing gas at a CA Chevron station, and in a full on rain storm.  As we continued on the trip and it warmed up outside,, it dissappeared... never to occur again.

  I now have an ocassional hiccup just as described when driving around town at low speeds... just a touch and just like Wes describes - here in the arizona heat with no moisture.

  Madeline