Your computer may be overheating if there's a random slowdown. I have the game too, and I haven't experienced any slowdown on any specific levels at all. Does it only happen on one level, or does it slow down at a certain time after you begin? I've got:
Pentium 4 2.8GHz
896MB PC3200
GeForce 6600GT 128MB
Runs fine on my computer throughout the entire game.
EDIT: Are you using ForceWare drivers? If so, you might want to consider switching to other drivers. I used to have some horrible speed with CS: Source, then a friend directed me to Xtreme-G drivers for nVidia Cards, boosted my Framerates up by like 10 - 20 FPS. Check it out...
I believe the site was
http://www.tweaksrus.com
Jam it back in, in the dark.