FPS drops since 24 feb update

As title says, I was thinking it was some process on my computer occasionally eating ressource but after running a game with the process list on the second monitor it looks like during thoses drops the game uses only one core (50% usage on a dual core), while it uses both most of the time. It doesn't last long, maybe 5 or 6 seconds but it sure is annoying.

Quick specs if it can help:
Seven Ultimate 64
Intel Core 2 duo E8500
8g ram
Nvidia GTX285

Same here, but I can't see my ressources with 1 monitor so I don't know if it's the same with the one core / dual core.