Really Bad Lag

Phin

<div class="IPBDescription">In steam</div> Fairly recently I've been getting rather large lag spikes whenever I encounter a marine, (as an alien), or vice-versa. The game's fine when they're not in my line of sight however, and it doesn't seem to be effected if there's a bunch of teammates instead of enemies. I'm playing v2.0/2.01 and I'm using OpenGL mode.
This never used to happen, so is this a new problem with Steam? Or is it just my PC starting to go on the fritz?

Win XP Home
Intel Pentium 4
1.70 GHz
640 MB of RAM
GeForce 2

VeNeM

same happens to me, fps is the same, just turrets or whatever make my ping skyrocket.