Garry's Mod Crashing

After playing GMod for about thirty minutes to an hour my computer will either freeze (and I have to manually reboot) or my computer will blue screen. I spend most of my time online playing TTT.

My PC specs:
CPU - Intel i5 3570k ivy bridge @3.4GHz
GPU - Gigabyte AMD Raedeon HD7870
RAM - CORSAIR Vengeance 8GB 2 x 4GB
PSU - CORSAIR Enthusiast TX750
MoBO - GIGABYTE G1.Sniper 3