FCLK is responsible for connecting the PCI-E controller to the rest of the system agent/ring bus.
Most parts will not be stable at 1.5GHz FCLK and you cannot stress/test FCLK with CPU/memory tests like Prime95 as they will never touch PCI-E. Errors on PCI-E from an excessive FCLK could easily cause GPU related issues.
...rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law,' because law is often but the tyrant's will, and always so when it violates the right of an individual. -- Thomas Jefferson