I have seen some drivers incorrectly identify a cards core and memory speeds
and overclock the settings. That will cause your issues. But if it's been running
like that for awhile it may have already damaged your card. Memtest won't
help you if your card is artifacting in games. I hate to say I'm right, but I am.
If the drivers don't help you need to follow the steps I previously posted.

Edit:
And those steps may or may not help. The gpu can usually withstand the
extra heat from overclocking or a faultly fan. It's usually the memory chips
on the card that will crap out 1st. But if your lucky :-)

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