If I remember correctly, this is a server-side issue, it's happened in the past but Garry patched it away and I guess it's back.
[EDIT]: I'm pretty sure it's about having too many indices (materials, sprites, etc) on screen. I'm pretty certain only Brass can fix this on this side of the issue, unless I'm 150% wrong.
[EDIT]: I'm pretty sure it's about having too many indices (materials, sprites, etc) on screen. I'm pretty certain only Brass can fix this on this side of the issue, unless I'm 150% wrong.