I had a very similar problem once. Took me days to figure it out. I'm not saying this guy did something similar, but it was a lesson in humility.
I had sat my brush on the plinth, just next to the pivot of the tone arm after cleaning a record and started to play. About halfway through every side it tried, the records would skip. Oh, God was it frustrating. I looked at everything reset everything, couldnt figure it out.
I was about ready to send it back for warranty work, when I dont know how how I realized it but the tone arm weight was just barely bumping the brush. The brush was heavy enough to not be moved and cause the tone arm to skip, over and over.
I was so embarrassed. I never even thought the brush would have caused this, I was so convinced it was something bigger. I honestly didnt even see the brush there.
Anyway, just wanted to share that becuase I'm sure everyones been there in some respect.