I got a 2001 yz250 and I just put a new top end, crank bearings, transmission bearings, shift drum bearings, and replaced all engine bottom end seal for crank, water pump, outputshaft, etc. I get the motor back together and it shifts through all it's gears fine. Sadly I think I'm going to have to crack it apart again because 1 gear is there but it wants to keep popping in and out of the gear but 2nd 3rd and rest up does not do this. I have had this motor apart and as far as I know there can be any issues with the shift drum or shift mechanism or shift pause. All those components seemed in tack and now wear noticable. I did not inspect the transmission like I should of. When we got this bike it was not running and the owner said he didn't want it he got it from a friend and he didn't have time for it. We paid 200 for it. My question is would you think there's worn out cogs on the gear? I wanna be led in the right direction in solving this. Any help is greatly appreciated. Thank you!