A couple things to try:
-driver sweeper then reinstall drivers.
-check bios settings for PCI-E preference. Perhaps locking in PCI-E (if on auto) will fix it, perhaps unlocking PCI-E (if it is set to use PCI-E) will fix it. Just a shot in the dark.
-test the card in another system. There is always the chance that during the swap you damaged the 275.
-test another card in the system, preferrably a powerful one. If there is an issue with the PCI-E slot, the slower card (7600gt) might not be triggering the issue.
-Update motherboard bios. This is a real shot in the dark.
AND THE HAIL MARY GRAND FINALE;
-format your HDD, reinstall windows