01 YZ 250 wont shift to 3rd

I have an 01 YZ 250. Periodically it won't shift up from 2nd to 3rd. Never had a problem going from 1st to second, just sometimes the gear shift lever physically won't budge and go up. After trying to upshift for a couple of minutes it will suddenly be fine and shift up to 3rd and 4th like nothing was wrong. Any suggestions?

I am running Maxima 80 wt and form reading posts on this site, it seems I should experiment with running different oils to see what my bike "likes"... Oh yeah I also just installed a magnetic drain plug.

Help...

Thanks,
Rob
Edwards AFB, CA