Another problem that apparently can create this symptom is a worn input shaft bearing (might not be the exact name for the bearing) in the transmission. I've seen it mentioned a number of times over the years in articles and forums, it typically happens when there is a lot of torque on the input shaft of a standard transmission, the only cure is to replace the bearing. Years ago I had a 1983 toyota 4x4 that would literally jump out of first gear while accelerating, I had just enough money to replace all the bearings (5th gear bearing was also going bad) and that cured the problem.
Don't know if that's happening to you, but just wanted to raise the possibility.
