On any gamemode Garry’s Mod lags!!

  When I play it starts with 60 fps then drops 10 fps, then 60 fps then 10, and so on and so forth.

I’ve gotten quite sick of it and I’ve died so many times because of it. Compared to all my other games
this one has the only problem like this. Counter-Strike: Source flawless 100 fps. Garry’s Mod makes me so
frustrated with this problem. PLEASE HELP!!!

System specs?