I'm getting ready to do so searches on the list but here's what is
happening.

I have a 84 manual with a 2.1 rebuilt in it.

Last night while driving 65mph it pops out of 4th gear.  I put it back
and run a
while.  It pops out again.  Slowing down and accelerating seems to make
it pop out.

There is movement in the shifter if the whole transmission and engine
move
slightly when accelerating or slowing.

Maybe I need a new transmission mount.

I have all new engine mounts less than a year old.

I think the bushings around the shifting rod next to the transmission
are worn out.

No other noises, no hard shifting.  Clutch, pilot bearing etc seem
fine.

Last weekend I bled the brakes and put new hand brake cables on.

Is this the 3/4 slider gear problem?  If so were should I order a
rebuilt transmission from?

Honestly this is a kick in the pants.  I have been making an effort to
get as many of my
buses in the best shape possible and this is going backwards.

(moral support plea.)

John.