Popping out of 5th is a sure sign of either one of two things.

The 1st is lack of transmission fluid. When did you last change it? I would suggest first getting underneath there and checking for obvious leaks. Your oil seal around the CV joint could be bad. Replacing that is only a matter of a few dollars. Do the flush first and drive it, checking to see how it runs.

The only other experience that I've had with the transmission popping out of gear is synchronizers. If that is the case, it is time for either a rebuild or new transmission.