I had to split the cases on my '81 xr200 back in the day. That problem was it was hanging up in second gear. Problem was the slot in the shift drum had worn a burr and it was hard for the shift fork to slide properly. Not sure about y our problem, but it does sound clutch related. If it was something in the tranny, it wouldn not ahve let go just sitting there at a stop

FWIW, i got a Clymers manual and Ithink that special tool to take the clutch off (looks like a 7/8 socket but has 4 prongs on it. I think I made it with an old socket and a grinder.)
Mike