Gearchange sticks when hot

I rebuilt my V50 III with Monza footpegs and gear lever. The bike hadn’t been used for some years, but I had no gearchange problems before the rebuild. The gearbox was not opened up as part of the rebuild, but I did take off the breather, and found the detent plunger was quite worn on one side, so have reground it to symetrical profile. Now the bike changes gear no problem for about 12 - 15 km until the box is warmed through, then starts to get ‘sticky’ between changes, and will eventually get stuck in gear - usually 3d.
My next step will be to buy a new detent plunger, but any better ideas welcomed.

Always a good plan to look at what you did last.

check the clutch properly.

classic symptoms of it dragging, check enough free play and rear piviot lubricated and free

also should be between 900ml and 1 litre of ep80/90 in the box.

lube the linkage system , esp where it goes through the frame .

I found that adding some “liquid molly” to the gear oil helped a lot. cheers, Chris