I know this answer isn't what you wanted but perhaps you should take it to a mechanic, or someone who would have a better clue on what it is if they ride in the car with you. If it only happens when you're in the fifth gear, it may have something to do with that specific gear, thus the transmission. My fifth gear went out with a hard "kick" and I wasn't able to use it until I got that gear replaced.
Honestly, though, look at what speed you're going, what your RPMs are, does it happen when you first put it in 5th, or a little after, etc. All of these things can help you, and someone more expertise figure out what's wrong. Finally, a TRUSTED mechanic simply looking in the transmission or other parts of your car may be able to just see what's wrong.
If you know how to check your transmission fluid, that could help determine if that is it.


