RB300 maintainance...

The phono cable that attaches to the arm base could be under tension at the disk extremes. I know nothing about your table so excuse me if this is not an appropriate place to look. On my table the DIN connection at the arm to the plinth needs enough slack to allow the arm to pivot without any tension and can sometimes be difficult to adjust.

I think you need a set of Denessen Cartridge Protractor to porperly set-up your tonearm/cartridge.
Do you have the same problem with the Benz cartridge?

...now I have some info to think of... With Benz it's even more emphisized than with Lyra. So far I didn't check the Lugnut's thought and I'm going to work on that.

Now we knew the problem didn't caused by the cartridges.
I don't think the problem is relating to the anti-skate, cable or anything.
I'm sure the problem is caused by the misalignment of the cartridge or even the arm was mounted incorrectly(I mean the position) or both.
Does this problem happens to one particular record or it happens to all your records?
Try to put the cartridge at the end of the record. Is the cartridge parallel to the groove? Now try to put the cartridge at the beginning of the record. Is it parallel to the groove again? For a properly align cartridge, the cartridge is always parallel to the inner edge(the end) and the outer edge(the beginning) of the groove.

As I sugguested before, you need a Denessen Cartridge protractor to properly align the cartridge and arm.

Make the following arm test:

Place the stylus guard in place, and put the arm in perfect balance at zero grams, the anti-skating at zero too. Then slighly tap with your finger the arm to slide it towards the spindle and to the back, to rest position. The arm should be travel free as a feather. If you noted hesitation or any anomalies, unfortunally the bearings were damaged in a degree accordingly to the anomallly.

I hope this is not your case, because there no more could be done to fix it, only the manufacturer.