I would have gone for the bush option after all else you've had done. How old are the current poly bushes? I have had to have brake bushes redone in the past even though they were polybushed. Its worth double checking.
The other option is a warped disc but I can't see how you would of warped them as you were nice to them on the bedding in, unless they were faulty to start with?

Bit of a mystery, hope you get it sorted!