If you're under CPO: Remove your mods & have the dealer run a diagnostic. Even better, if you can try to have a tech ride in the car & experience 1st hand the shifty 5th gear change. Hopefully once they identify what's wrong, they'll fix it with no major costs to you.
If you're out of Warranty: Try to find someone with a BAV Tech/INPA scanner check for Trans related fault codes. Hopefully the 5th gear isn't torn to shit (no idea how, would expect 1st/2nd to be beaten up, not 5th) and it's a simple fix