I recently fixed a problem many people here ask about: an annoying skip in the cassette when shoving off or when standing on the cranks. After searching BF, I did all the usual things: replaced the cassette and chain, checked that my derailleur hanger wasn't bent, checked that the derailleur was well-adjusted and aligned with the cogs in each position. No luck.
So I just lived with this for a while... then I decided to replace my wheels. The rims were chewed up and the hub bearings and races had seen better days. And my shifting problem disappeared!
On my old rear wheel, there was some play between the freehub and the hub body. I think a lot of weight caused the freehub + cassette to move slightly--enough to cause the chain to begin to shift, but then move back. That's my theory anyway.
Just another thing to check if you have this problem. Hope that helps someone.