Problems with acoustic recording in Garage Band

I've been using Garage Band to record an acoustic piano (a real instrument!) through microphones via an Eridol FA-101 Audio Capture system. However, the playback quality is bad - there are brief dropouts and crackles, but the worst aspect is pitch wobble, the sort of effect you'd get if you were playing back a tape cassette that was slipping on the playback head. The problem is not with the audio capture system, as I get the same effects when I use the built-in microphone. Is there some critical setting that I've missed, or am I asking too much of the program (or the computer's processor)? Many thanks to anyone who can help with this.

A quick update on the above problem - I tried importing the recorded track into iTunes to see if that playback quality was any better - with no luck; the sound was just as bad. However, I then tried burning the track onto an audio CD and played that in my (non-computer) CD player. The quality, while not perfect, was much better. I assume that the problem must be in the computer's playback system. In the end, what I want is a good quality export file, but it would be nice to be able to listen to the recording at final quality from the mac. Any ideas?