Strange singleplayer map problems

Hi, lately Ive been having some strange problems with downloaded and game maps (like the ones from the actual games, HL2, EP1, EP2 and portal). When I play them everthings fine for about 30 seconds then it start getting laggy as hell. Ive got no idea whats causing it and it cant be missing textures as Ive checked for purple and black patterned things and there are none. I cleaned out my Gmod but that didnt solve the problem. It plays fine on maps like gm_construct and flatgrass. Ive tried turning my graphics down but that didnt help, Ive also checked and updated my Drivers. If anyone can shed some light on this problem I would be very grateful.