I've installed 2GB RAM tonight into the machine I'm playing the game on and low and behold... The lag spikes are still there.
One minute I'll be running along and the next, I'll appear in a totally different place, sometimes inside building I was outside a blink of an eye ago.
FPS is averaging 30-50 with nearly everything on 'Max' but with HDR Bloom turned off. I'm running the game on a Windows XP Pro machine and my ping is usually very low, as the connection is 8mb.
All that said, the lag spikes are still there and it is somewhat annoying, especially if I've just got myself into a nice 'sniper position' and suddenly I've moved 20 metres and I'm laying on the ground under the nose of an enemy.. !!
I trust Tripwire will look into this with seriousness (they've been on the ball so far, in general). I look forward to a swift resolution.