No I never had any problems with gear slipping with my GTX gearbox. No linkage adjustments were necessary either.
But I did do a gearbox oil change as a preventive measure. I actually changed the gear oil, ran the vehicle for 500 km, and then did another gearbox oil change with full synthetic Mobil 1 gear oil.
Just one other comment that may seem a bit silly, but might be worth a look. Are you sure that you are fully engaging 5th gear? Do you push the gear lever all the way up into 5th gear? Try this: sit in your car with the engine off, and cycle back an forth from 4th to 5th gear, pushing firmly up to fully engage 5th gear. Do this 50 times or so. It might free up a sticky or rusty gear cable.
