GrayRacer this my old trans it worked fine it didn't jump out of gear.
I put a WR trans in my YZ and it was just sitting on a self in my garage before I gave to Dennish87.
This why I think it not the trans I could be a shifter fork.
The shift fork he used is new, in case you missed that. Only the center shift fork is implicated in a fifth gear problem.
Chain and sprocket problems would occur in all gears, and the bike would be more prone to forcing the chain to jump in low gear than any other because of the increased torque multiplication in the lower gears.
Obviously, without riding the bike myself, I can't say for certain, but it sounds classically like the trans, whether it did it for you or not.