What would cause my manual transmission to pop out of 6th under acceleration.

What is your question?

Took it in for repair. Explained when begin accelerating to 6th gear, pops out and into neutral. Than few weeks later would not go into or out of any gears. They replaced the clutch put it back together and when they took it for test drive, problem with 6th gear. They now inform me I need a new transmission.