Mbox Performance Issues? Options

[meadows.83]

i have been using my mbox/ibook g4 800 for about five mos. with no problems up until about a week ago, i started getting a message saying cpu usage is holding off usb audio, increase h/w buffer or remove plug-ins. i get this when my h/w buffer is already as high as it can go, and i'm only running like four tracks with no plug-ins at all. how could this be? i'm running osx 10.3/pt le 6.2.2

i'm not using an external hard drive, could this be part of the problem?

thanx