Sounds very much like the issue I had recently.
The reverse lockout lever on the gearbox needs to be in exactly the right pace or 5th 6th and reverse gears will just begin to act wierd, crunching and selecting the wrong one. There is a cable that goes down to the gearbox, very much like a throttle body cable, and it holds a lever in the right place. Sounds like yours needs adjusting.
There is a hole to line up on the lever and the box. You adjust the cable length to get it in the right spot. I used a drill bit to align things while I adjusted the cable
alternatively, your gearbox is farked