Hey all,
 
I recently got this on Steam when it was on sale, it ran fine for the first couple of weeks but now certain combat challenges have awful frame rates. I did a check and my average FR is 59, on certain challenges it's less than 10 :( I've tried altering the settings and it doesn't solve anything. 
 
Has anyone else had this problem?

Strange since all challenges are basically scenarios from the main game, perhaps with a few extra henchmen thrown in. I had no problem with the challenges. 

Agree with Rockanomics, its probably physx, what are your specs/settings?

I'm gonna break away from the crowd here & say it's your PhysX. That screwed over Mirror's Edge for me so bad.