Very strange distortion problem in garageband

I am using an external USB sound card plugged into my MacBook Air to record my synthesiser.

It works okay for the first take (as long as it isn't a partcularly long take), but upon trying to record subsequent takes heavy distortion suddenly sets in. Unplugging the sound card and plugging it back in again fixes this issue, again for just the first take.

However, if I use Audacity, everything works absolutely fine. This is a Garageband problem.

However, a strange thing I noticed:

My soundcard flashes when it is active/transferring data.

From the minute Garageband is opened until it is closed, the sound card is constantly flashing.

Whereas in Audacity the soundcard only flashes during recording, and does not do so between takes.

I don't know if the way Garageband appears to keep the soundcard permanently active has something to do with it.

Apple has annoyingly gone downhill with stupid little problems like this. I'd like to find a solution as Garageband is a nice program to use. These are the kind of pathetic glitches I'd expect from Windows, not Apple kit like this.