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 also had 344.75 and was getting pretty much 100% crap. Upgraded to 347.25 and running with -x 13:
Not exactly great but seems a bit better. Trying to get hold of CUDA 7.0 to try it out, might be a while.
Also made the found block notification a lot less noisy in the latest release, that huge dump was intended for debugging and I just didn't realize it was getting printed even when not running with -D.