|you identify that the issue appears graphics related, and i'd agree. you can tax cpu/ram with software like Prime95, and test graphics with something like 3DMark06. You said games, so i'm assuming you can tax your cpu/ram without the system crashing.|
you said you think your temps are okay, but where are you obtaining this information? it could be temps, either WAY too high right now, or a little too high over the last 18 months.
did you build this machine or was it professionally assembled? you didn't say much about your PSU, so if it's crappy it could have been wearing down the life of your GPU over the last year and a half. Power surges and brown outs could do the same thing.
not to mention you said your card was OC'd. did you do that or did it come that way from the factory? either way, that will wear down the life of hardware faster than normal, so whereas you might have gotten 3 years out of it at stock speeds, it might start to show its OC'd age after 18 months.