Dropping out of gear at high rpm

[Maj]

First to second is more common with worn dogs and or bent shift fork but any of them can do it

Be sure your shifter is not contacting anything limiting its stroke , and it may be worth a look in the selectors to see if the mechanism (detent and pawl )is all operating correctly , i think its more likely in this area than forks and dogs if its going back to 5th

5-6 uses 2 forks and gears on opposite sides of the box , its a slower shift , and any time i have not selected 6th properly have found a false N , sort of whats making me think the shifter and not further in the gearbox