After taking another look at this thread I'm inclined to lean more toward the freehub binding. If it's not turning (backward) freely, then the chain is going to tend to bunch up on the chainstay. Just as the OP states. Just remove the rear wheel and spin the cassette backward, by hand. If it is binding it should be pretty obvious.

So, I'm still in the "clean & lube" frame of mind, but it's more likely that it's the freehub that needs the servicing. I'm pretty sure that if you do a search you can find a service proceedure for CK freehubs.