This could be either a simple heat issue ( which i think it is) or could be a case of faulty memory
Check the temperatures of ur CPU in the bios ...n' post 'em ..
As i understand from ur post ..ur system is 3 years old ..n' if there has been dust accumalation on the Cpu heatsink fan ..it could lead to increase in CPU temps ..Hence the freezeing of the system.
What i would suggest is that u clean the CPU heatsink Fan .. probably apply a new layer of heatsink compound(artic silver)
N' as an immediate solution ..leave the side panels of ur Case open.
Alternatively .. if u don't find the temperatures high ..then ..u can try by removing one of the memory modules (i'm assuming that there are 2 ..) or try out with different memory
*edit*
Please post ur system specs the nxt time u post