hmmm i had gmod for my old computer and i would have somewhat similar problems
in my case i would be actually playing and the computer would randomly turn off and turn back on again. I wasnt able to fix the problem, until i got a new computer which runs it fine.
I really dont know what im talking about but i would agree witht the replies on the steam forums, it is probably your video card. Do you have an integrated video chip? (the crappy ones that come stock in most home computers)