Reply to Logic Pro with External sound card

Hi!

I am trying to set up my Logic Pro with my external sound card Lexicon Alpha.

I start Logic, and everything seems fine. It sounds fine... But after aprox 1 minute the sound that comes out of my headphones sounds sooo bad. It really sounds like the sounds is broken. It sounds good for a minute, than breaks down.

I guess this is a common problem, and I have searched Google, but that didn't help me.

How should I set this up?

/ O