FPS drop while particle effects are on screen

Hey there,
I've been having this issue for a couple of weeks now.

Whenever there are a lot of "effects" on screen, i.e. infestation (rich or minimal), rail gun beams, gas and especially bilebombs, my framerate drops a little bit too much.
The game is running with 60-90 fps without any of those effects going on. For example, if I bilebomb something (and stand close to the "eplosions") the framerate drops to about 20-30 but feels like 5-10. Also, I get an annoying input-lag, similar to an enabled v-sync (it's not enabled). Also, the framerate drop is a lot more pronounced when standing close to where the bilebomb/beam/spike is at the moment.

I know that a framerate drop is expected when a lot of action is going on, but this is just a bit too much and I haven't experienced it in the builds up to 247.

I already tried the last three driver revisions + beta drivers.
In-game options should be fine, I can post them if needed. The problem occurs regardless of the graphic settings (from 640*480 LOW to 1920*1080 HIGH)

If anyone has an idea, I would be really grateful.


Windows 7 Professional
AMD Phenom II X4 @ 4,00 GHz (OCed from 3,6 GHz)
Radeon HD 6870
8 GB Ram
Game installed on SSD