your box sounds pretty much shagged! maybe take it to a specialist to be sure
My 4 speed pretty much had no reverse, if i revved it hard it just sat there, if i just touched the pedal it would slowly build momentum. sometimes it would go in to drive, other times it wouldnt
Took it in for a check up and the guy was surprised it was still driveable, he said a few of the solenoids are shot, seals are hard and the thing was pissing out oil in every which direction
Get it checked out but I think it may be time to do that 5 speed conversion