Ran across this by accident. I believe there have been reports of the transmissions jumping out of 4th or 5th gear. Looks like there was a parts upgrade to solve the problem.
http://car-and-safety.com/service/mitsubishi-fuso-fe639-9999.htm#POWER TRAIN:MANUAL TRANSMISSION
http://car-and-safety.com/service/mitsubishi-fuso-fe639-9999.htm#POWER TRAIN:MANUAL TRANSMISSION