Rainbow of Death

if i join a server in Garry’s mod, it will take forever to send client info, and after i join, it will freeze for a minuet, then i can play for a few seconds, then my screen will have random triangles in random colors all over the screen, then it says hl2.exe has stopped working. in single player, it does the same thing, only if i press ctr alt Del and the click the g mod tab it fixes, but then does it again after a few seconds. Any help or suggestions?

Is there any way you could get us a picture?

System Specs? Addons Installed?