I put the adjustable detent screws in mine and before i had them adjusted properly it would pop out of gear when i let off at higher speeds, tightened them up a little and good to go so far.
It could be wear on the shift rails or maybe the springs that hold tension on the detent ball is fatigued..
google twin sticking a 205 for pics of the internals, 205's are pretty simple, theres only so many things that could be wrong and 99% can be fixed easily