What is the spec of the rest of the pc?..is it connected to the internet and if so how?
If it runs ok when gaming for a couple of hours at a time then it's unlikely to be overheating....but always worth checking that the graphics card and cpu fans are working/not clogged w1th dust.It's also worth checking that you have the latest drivers for the graphics card,motherboard chipset and soundcard installed.
Do you get any similar crashes if the pc is used with other programs over long time periods?
An example of a game that crashes along with the settings he uses (eg screen resolution ,high quality graphics setting,any other setting used) would help.
There are a number of applications available which will test your pc to the limit and weed out any errors but I would like to know more about the pc,games that crash and what happens when it does crash before we go further.