Have you got fluid in the gearbox? (or enough MTF94 in there at least)
Had similar where my car would push itself out of 5th gear (180) on motorways... turns out it was no oil in the box, cost me £400 at the time to get the box rebuilt
- Only for the box to start doing it again 1 year later with the same result... no oil!
In tat case it turned out to be the selector shaft seal that was faulty, I changed that myself for £5 and then filled it up with oil again and had no issues with that box till I sold it two years later (that was the blue 180)
Anyway... check the oil levels
