How much oil are you putting in the drive shaft? If you put the book quantity in (150cc) then when you drive fast for long periods of time the universal joint slings the oil through a vent hole between the tranny and the drive shaft and fills the tranny. Use only 100-120cc of oil in the drive shaft will stop this migration of oil. If the tranny fluid is not overfilled then the only other place the oil can go, assuming it is not leaking out the drain plug or universal boot, is into the final drive past the seal between the final drive and the drive shaft. Replace this seal only then. Overfilling the final drive with oil from the drive shaft might cause problems much like you are seeing.
So I guess more investigation is in order.
Good luck.
