Excellent choice the Twists Xellex! They are my favorite too because it gives the beetle an outstanding finishing....!!!
From the photos seems that the bumpstop is too close in the trailing arm and i assume that this is your problem.
Before you cut it,just remove it and take the beetle for a ride to see if it repeats the problem again.If not then cut the bumpstop at least the half and you will be ok
Steve,
This tyre pressure it is for 17" rims? Because i use for mine 30 for rears and 26 for the fronts, but i dont know if it's the correct one..