Strange trans problem. What actually keeps it in gear?

[Tedskiii]

Update;

Still having the same problem, it happens randomly. Lastnight it slid out of 4th when I was down shifting from 5th.

Also another problem has came up about two weeks ago ..

Whenever I shift above 2.5k I feel a strange feeling, and small noise. It's not a grinding sound at all, it's hard to explain, but something clearly isn't working/engaging right down there. My possibilities/questions.

-Could this have anything to do with my trans fluid? I've read stories of people fixing this with a simple fluid change. (I'm using Royal purple because I couldn't find the Redline in stores)

-Maybe my pilot bushing was put in wrong? (I installed it all the way in. Is it supposed to be flush instead?)

-Maybe I didnt seat the throwout bearing properly?

Everything down there is brand new except my clutch fork.

Any thoughts on this?