Gmod connection problem

ive got a weird problem everytime i want to play gmod online it crashes and it wil say lockvertex bufer somthing engine eror and then i must reinstall the gema otherwise it will happen again

so i can only play gmod in single player

PC specs?

pff idk exactly but i have gmod for an half year or somthing and this never happend before
until 3 weeks or somthing what i do know is that my video card is a bit old and i get a new 1 but i dont think that that is the problem

cuse it only crash when i want to go in multplayer