2001 A6 2.8 vibrating

Hi - am new to this group. Bought a certified preowned 2001 A6 2.8
last fall. At that time the car vibrated at and above 65 mph. They
fixed this with a front end alignment. In Feb. I blew a tire on a
pothole, resulting in a bent rim on what is now my spare tire. Got a
new tire, all tires were balanced, front end re-aligned, no problem.
Then a couple of months ago the car started vibrating at 65 mph again.
Took it back to Audi a week ago, got a tire balance, no problems. Then
ONE week later, it started to vibrate again at 65 mph. Obviously
something's going on to make the car vibrate again - and it's not like
I'm off-roading with the car or anything! I'm just driving it on
normal highways. Any suggestions of what I can make sure Audi checks
out? TIA!