skipping

I have now got my pc set up as a transport. Using lenovo t60 with windows xp. burning to harddrive using Exact audio copy to compressed flac files. Playback with foobar and asio. usb out to poppulse pc link. pc link to northstar dac ....etc.
unfortunately, I am experiencing lots of skips when playing back the files and don't know why this is happenning. Any thoughts or suggestions on remedying this would be appreciated.

Thanks