During CS:S when I'm running about, maybe not even shooting at an opponent, my fps will drop from 300 to 50, when this happens this causes major lagg, my game may jitter for 5 or so seconds, or just plain freeze for 5 seconds. When I hit win+r, and go into "msconfig" and go to the area where I can activate cores and maximum RAM etc, it only let's me chose one core when I have four;

msconfig; http://img580.imageshack.us/img580/4905/fmly.png
Processor; http://www.pcworld.co.uk/gbuk/amd-phenom-ii-x4-955-black-edition-processor-04820900-pdt.html

ul hope u fix it.

Dankeee

Had this issue a few days ago, things that helped initially were turning off AA and texture filtering but that was only a temporary fix for me. In game go Options -> Mouse -> Enable Raw Input. If the option is greyed out then put "m_rawinput 1" in to the console.

Hope you fix it, its such a pain in the arse :\ <3