OK - Ive installed the latest drivers for the X1900XT and its made no difference.
For those just jumping to the end of the thread;
Overclocked the games play perfectly offline - but are jumpy when played online.
At stock speeds both online and offline gaming is good, with no noticable graphical difference between them.
So - Whats causing the problem!?
So far ive tried:
Reinstalling game, reinstalling drivers, setting cpu affinity, using new NIC card, and reinstalling windows. - None of these have helped.
Im quite annoyed as i spent that bit extra on buying very overclockable components...
Any more ideas guys?