If it's not what Balsta suggested, it could be this:
I suffered similar symptoms. From time to time, I would experience very stiff gear changes. It would effect all gears. If I stopped and turned the motor off, upon restarting the gear change would normally be back to normal.
Quite a few miles later (around 17,000 miles), it jumped out of 6th gear, and from then on I could not select 6th or change down from 6th without great difficulty, and a fair amount of crunching. I found a chip of metal from the selector drum on the magnetic drain plug.
I think one of the rollers (no. 4 below) had become damaged, thus causing the stiff gear change. When the roller rotated, the gear change would return to being good again. Eventually the roller caused the wall of the selector drum to fracture - this is when I lost 6th gear. I replaced the damaged selector drum (with a stronger upgraded part - that your bike should have) and the damaged roller. Now all is well.
This is how I found one of the rollers: