Light map/shaders glitch

I’ve been having this problem recently where my shaders and lightmaps(i think, im not too sure what the problem is) will spontaneously stop working, and the colours will screw up. This carry’s on in the main menu, and the only thing i can do to fix it, is to restart gmod.
Can anyone help?

This video demonstrates what happens, but it is not mine: http://www.youtube.com/watch?v=mqDyPcKYkB4&feature=related