400/4 fourth & fifth gear slipping out under power... :os

[470four]

Hi all,
Got the above prob with my Four... when ridden "briskly" fourth & - I think - fifth as well slips out into a false neutral, would this be a selector fork issue? 

[Bryanj]

Probably but also when they slip the "Dogs" and "Holes" get rounded edges so the relevant gears will need replacement, which will not be 4th and 5th probably

[470four]

Granted... anybody have a spare 400 bottom end they dont need?