I just went through the same thing...still in the process of putting it back together(just waiting on my jug and head from Todd).Mine was jumping outa gear under full power...3rd,4th,and 5th were all doing it.
With mine, it was a combination of gear dogs/forks/drum being a little worn...sooooo,I took the high road and dropped $1200 on a complete new tranny....but the kick in the nuts over the whole thing, was that when I pulled my jug off to start disassembling the motor, I found out I had a cracked intake bridge
...so off to Pinnsonault the jug/head went, since he's only a little over an hour away from me.
As of right now...I'm about $2400-$2500 into this motor rebuild...with new inner clutch hub/pressure plate/clutch plates and springs,cylinder studs/nuts,gakets..etc,etc....oh yeah,I'm balls deep into this one!
Sorry if this wasn't "soothing"....but, as the old saying goes...."expect the un-expected"!