I dont have any experience with Mobys, but your problem sounds pretty common. You're right about the heat thing...most likely its your transmission fluid. If your fluid level is low, the heat will cause it to burn off and lock up the 'ped. I would definately check the trans. fluid level and change it. Drain all the fluid and add NEW transmission fluid. Also, it could be the clutch, I would check that too. Like i said, I dont have any experience with mobys so I could be off the mark here, but it sounds like a generic problem.
Hope this helps