Minecraft is lagging on my pretty decent computer

Minecraft is lagging on my pretty decent computer! Help?

Added (1). So I used to run minecraft at a solid 120FPS (locked) but now I'm usually 40FPS and below. What gives?! I have all my drivers updated and my drives are defragged. Any ideas on how to help?
Specs:
OS: Windows 8 64-Bit
CPU: AMD 8320 8 Core @ 3.2Ghz
GPU: Nvidia GTX 560 2GB Superclocked
RAM: 8GB 1366mhz RAM
PSU: 600W Corsair

Stop background running apps.