I had the same problem with the Vukas. The only way I could prevent slips was by using lots of Tacx assembly paste and tightening beyond the recommended torque.
I do that AND I have two zip ties looped through the pad holes to help stop them moving down. It's pretty solid but eventually will move. In fact, after the TCR I found the zip ties had broken so I hit some pretty big bumps.
My "slipping" setup is Zipp Vuka Alumina bars on my ultra bike.
My TT setup is completely different.