Adventure Gearbox - Knackered?

Recently noticed bad vibes when the bike is above 4 thou. mainly in 4th & 5th gear. Seems to be acting as if its being 'slowed' down. It seems OK in 6th.

What are the common issues with gearbox problems on this model.

Bike has done 24k and just had a full 24k service at RGM.

Any ideas?

Thanks
Sam