Quote Originally Posted by HiJon89 View Post
As the OP has stated multiple times already, the same cards that fail will pass the test when sufficiently underclocked, suggesting a hardware rather than a software problem.
Suggesting, but by no means proving.

Assuming that there aren't any bugs in the software, testing if it is a PWM problem or a cooling problem should be fairly easy. Take a card that fails and increase it's cooling capacity.