distorted sound using usb interface and Mac, help!!!!

Hi, i am pulling my hair out trying to sort this, please help me!!!

I am using a xenix 1622 mixer with the supplied uca200 usb interface to record on my new mac os x leaopard. I am getting a horrible slowed down distortion sound whenever i play the audio back, or indeed sometimes when i try to record a sound. I have used the same set up for years on a pc which worked fine, but since my switch to a macbook this is happening. I have updated all the relevant updates from apple. I am using the new leopard system. This is happening on any daw i use so it is not specific to a certain programme. I wonder if my usb interface is not compatible with the new macs? or is there some obvious setting on my mac i need to do which i dont know? I have tried changing the buffer size and latency/delay etc to no avail. Please help!!!