I think it's because of the server. PERPHeads literally eats my internet, I can't even watch a YouTube video ALT Tabbed when on the server. Meh, I don't really know, try to not be ALT Tabbed when joining the server and be sure to close all browsers.
Gmod uses alot of CPU if you have things running in the background and you are using too much your computer will allocate your CPU power to what is most important which is why it crashes or times out due to it taking too long to load.
Try closing anything you are not using or pre load the map.