it could be anything, the stress that modern-day games put on dated hardware (even my 4850 is showing its age a LOT nowadays) is quite significant. it could even be down to CPU temp or HDD temperature. i'm only speculating though. a good habit to get into is hoovering out your computer, so all the fans and filters are dust-free, and any components that are susceptible to dust build up (Power supplies are good at this). better yet if you have access to compressed air, that would be the better route for the purposes of cleaning a computer out :) who knows, but it's worth a shot right?