If it was running fine before, but now isn't, then it's not a problem with the game, but your computer.
Have you installed any new software recently that might be slowing it down? (e.g. antivirus, peer-to-peer, messenger)
Check your PC for spyware/adware/viruses. I recommend TrendMicro's Housecall.
Defrag your hardrive and make sure you have latest drivers.
If still not working, try uninstalling and reinstalling the game (unlikely to do much, but can't hurt to try).