Hey bojer..
I got the same problem with my galaxy xtr250 (same bike as Yours) i think??
If you look for chinaV's galaxy xtr thread he swapped out the standard gearing with a smaller chain 428, bigger front sprocket and smaller rear sprocket to get better road gearing on my bike, which helps a lot.. But without a balance shaft in the engine and the firm chassis it does vibrate quite a bit.. it still feels like it could pull a lot taller gearing too!!
I have a set of motox wheels with the 520 chain and gearing and i swap the 17 inch supermotard wheels and change to a 428 chain for road riding, i'll check the spocket size's next time i look.. unless chinaV or felix can remember.

rich