Soren,
I may have an interesting solution for you.
I remember that I had one of these turntables for repair a couple of years ago, and it was also distorting on only certain areas of the LP. I finally traced the problem to the 2 aluminium plates - the large one surrounding the platter, and the smaller one above the control panel.
They were not stting EXACTLY horizontally, and there was a small amount of friction between the plates, and the moving part of the arm.
It was caused by the fact that the chassis poles which stick up into the holes on the underside of the plates were not exactly aligned.
I solved the problem by taking a slightly larger drill-bit, and widening the holes on the underside of the plates - drill slowly and carefully, not to penetrate the platter's upper-side. Then the poles fitted into the holes perfectly, and there was no more distortion.
Regards
Menahem