Why are my materials all messed up?

Can someone tell me why this is? everytime i go into Gmod anywere from 5-30min. within me being in the game this happens…

What is your graphics card?

Post your PC specs below.

Nvidia GeForce 7600, Intel Celeron 2.40GHz Proccesser, 1GB RAM, 450WATT PS

anyone mind helping me?

Looks like artifact to me, your GPU may be overheating. It’s time to replace your graphics card anyway.