You could try slowing things down with CPU Killer :
http://www.cpukiller.com
Painkiller is a fast game in the first place, doesn't need things speeding up anymore!
Have you tried clocking the CPU down to around 1.5 Ghz, thats the minimum CPU speed required by the game. Also read somewhere about the cool and quiet function a lot of the CPU's are getting lately. You could try disabling that also, worth giving a shot to see if it works.
Final one I have found is alt tabbing out the game and getting up the resource screen (ctrl, alt & del) Task Manager. Find the game in the process list and right click and set the affinity to one CPU.