Audio: focusrite 2i2 - compatibility issues?

Hello,

I've recently upgraded my sound system with 2 KRK Rokit 5 G3s running via speaker cables through a focusrite 2i2. I'm running into a few strange issues with it though, and I'm stuck as to what might cause it.

Currently there are 2 main problems I keep having:

1. Lots of crackling/static type sound when playing music via youtube, itunes and when playing back in logic. I thought this might be the speakers bedding in, but I tried each speaker as a line out straight from an electric guitar and I didn't have any crackling issues. This also doesn't seem to fluctuate with volume levels.

2. Random tremelo distortion where the sound begins to become wavey and starts dipping out. This will be totally unprovoked and happens when [apart from problem 1] no other warning or problems are occuring. This only goes away if i disconnect and reconnect the focusrite to the usb port or if i turn the monitor speakers off and on again.

I'm running OS X 10.5.8, through the most up to date focusrite usb driver. Im using logic 8, but I'm finding the problem spans wider than just the music production side of things.

Is there anything that I might be able to do to resolve this? System preferences or anything like that??

Any help is massively appreciated

Rob