This does sound like a heat issue. We had a similar discussion about this same problem for the chevys just a few days ago. You can search for it posted by "Youther" called "Neutral Safety Switch" or you might find this link helpful ifyou decide to fix the problem for good.

http://www.novaresource.org/starter.htm

By installing a remote solenoid, heat wont keep you parked when you are wantin' to go!