Problem with Gmod

I’ve got a problem with gmod playing on any map except for flatgrass.

Basically, every second, there’s a split second freeze, just for a moment, just long enough to make a sound skip and a quick frame drop or two. I’ve seen it on every map i’ve played on except flatgrass. I play mainly with spacebuidl mods, but I don’t get what the cause of it is. Why would one map not do it and the rest do?

I’ve got the following specs:
RAdeon 5870
Q9550
4gb RAM.
Windows 7

No one else experiences this problem when playing on a server i host, and dont see this problem when playing the same mods I do when on a lan. Can anyone recommend what I should be lookign at to fix this?

And no… I do not have a pirated copy, just in case that was going to spring to mind, considering that’s apparently the default answer for any support questions on gmod.

Are you forcing any settings in the Catalyst Control Panel?

Nope, left them on “let the application descide”, should I be?

And it seems like it’s complete system lag, not video, because sound stutters too.