Hmm, strange ... I have 2 rigs with 970s - one with 6 cards and one with 5 cards. They both seem to find block on a regular bases without any rejected issues. Driver is 344.75
I'd say it's the driver, but I don't think I've mined spread with the older drivers and I had no rejects until a couple days ago. I set my 2 970s back to default instead of -x 13, but so far no rejects or accepts. Had 1 earlier when I had it at 13. Just shooting in the dark now though.