I think my GTX 560 is dysfunctional or something, because it's slower than my hd 6850..
If you are comparing like taking one out and putting one in then your 560 is it a GTX 560 ti of just a GTX 560? Anyway if your 6850 is warp speed and no laging then yes I would say it is your GTX 560 and it's bad and on it's way out.
If it is under warranty then RMA ASAP. At least you had another card to compare it with. A lot of people arn't that lucky and they go months with out figuring it out. No game playing for those people.
Let us know what happened and or what you are going to do?