The lag spikes within GMOD servers are quite common across most servers, Perp do have quite a bit of host sided issues but they are normally fixed quickly, this might be something client sided, could be something to do with your router, do you play wired or wireless? Also if your internet is fine it could be your pc, try giving it a clean with some sort of anti virus software, it used to happen to me all the time, I cleaned the PC and they stopped straight away. Hope this helps.