His game is rendering fine. Check the net_graph as it is still updating, this issue is internet related.
I sometimes had these stutters but they lasted a solid 3 seconds instead. It could be wrong network settings in your Garry's Mod or some weird routing issue to your ISP.
Disable your wireless/wi-fi and try only with a wired connection. See if that helps first.
Edit: It also wouldn't hurt to update your drivers. You can get them from your motherboard manufacturer's website (which are sometimes outdated) or the other method I prefer is to just look up the hardware ID in device manager. The top result is probably what you want, paste it into a Google search and you will get whatever network card you have on your motherboard if it doesn't have a name in device manager for some reason.
Don't download the drivers from some dodgy website. Get the from the manufacturer, in this case it would be Intel.