Mis-shift

All just picked up a clean 2000 KDX 220 unfortunately after i got it home i found out under a load or acceleration it misses a shift between 4th and 5th it into a neutral, It wont up shift at this point and you have to downshift to 3rd a come back up thru the gears under no load to get her into 5th and 6th, it has no grinding or clunks and no metal in the drained oil?? any ideas or had it happen before?
I think this is going to be a great bike and seems to run very well other than missing a gear....