The Update in NVidia drivers, access you to faster GPU data transfer rate. While on the older version driver, your game could not process's as much graphic, information to the card. via creating the more long term stable card, since less calculations have occurred. AFTER you updated the Driver, it has become a faster GPU thus creating better graphics, more calculations in the hours of gaming, resulting in shorter gaming time at the mhz you are demanding out of the card. why this matters and causes a card to stop working untill cools down is beyond me, but i feel it is possibly due to heated compositors and instruments on the card beside the GPU that need to cool in order to allow proper voltage flow once more, thus creating top speeds once more.
added note too. all BETA versions of anything are USERS own risk in use, in a way of explaining, lets just say they are not full developed, unstable files.
if you dont mind me asking i understand the SPU clock being 950MHZ but what are you overclocking in refference to the 1200/1900?
i hope this helps solve your issue.
My Rig Title : The in Home Satellite
MOBO: ASUS Maximus IV Extreme-Z
CPU: Intel Core i5-3570k @ 4.4GHz 1.1V
Ram: 4x4GB Corsair Vengeance kit @ 1600MHz 7-8-8-24
OS Hard Drive: 2x240GB Corsair GS SATA.III @ RAID0
Graphics Card: ASUS GTX770/DCII/OC/2GD5
PSU: Cooler Master 1000watt Semi Modular
OS: Windows 7 64bit Professional