My truck trans. round 2!

It seems like i might be having yet another problem with my trans.! IT had been working great since i rebuilt it but the other day only after it gets hot it seems to act up a little again not wanting to shift! But it only does it when it gets good and hot and it don't do it all the time just once in a while,could it maybe be a piece of dirt hanging up in it somewhere or could it be something else?

One other question kind of related,do these trans. have some kind of governor on it that won't let it over-rev?
I ask because after i kick it into passing gear and then if i put it right down on the floor it basically comes out of gear but if i let off the throttle it goes back into gear.
I don't know if it did this before i just noticed it the other day. Could this be yet another problem?