Pro Tools 8 LE Audio Problems

I have an iMac late 2009 with Snow Leopard running Pro Tools 8.0.3 with an Mbox2 and KRK Rokit5 Monitors connected to the mbox and a hub connecting the mbox to the mac. My problem is that i keep loosing audio on my krk's and I'm getting a pro tools message; CPU usage is holding off USB AUDIO. Remove some plugins....or increase the "H/W Buffer Size". How can I fix this issue? thank you,
Haqq.