audio glitches/drop outs with Guitarport/Gearbox

I'm using Guitarport with the Gearbox software. My speakers are plugged into the headphone jack of GP, and I'm using GP/Gearbox as my default audio. The problem is that whenever I do this, there are audio problems, such as glitches/skips/drop outs. However, when I plug the speakers straight into the computer's headphone jack, there are no problems. So is Gearbox using up too much PCU or whatever? There is supposedly a "Processor Performance" setting for the OSX, but it is not there. I'm using a MacBook pro.