Sounds like an isolation issue, that is over time your table is moving around a tad. Slight movements when you change the cart... just a thought. Btw I have the same table and my SDS is always spot on. I also use a 1000g clamp. Both the table and the SDS have their own stand. I can wrap the table hard with my hand while playing an album and you don't hear it...
Go figure.
Go figure.