My gmod keeps pausing and then eventually crashing whilst in RP servers.
This is a fresh install and all I have is PHX3 (SVN), Wire SVN
Specs are
Intel e8400 @ 3.6ghz
4gb of DDR2 1066MhZ
Windows 7 ultimate x64bit
Ati radeon 4870 1gb
19" Screen @ 1440x900

Any ideas ?

It’s not your hardware, I have worse hardware and I can play without crashing. It could be a problem with the server. Try a non-rp server and see if it crashes while playing.

If it does crash, then it could be your graphics drivers. What catalyst version do you have for your 4870?