Richard,
How many USB devices to you have connected to the computer? It's really possible that you're exceeding the USB bandwidth limitations.
My $100US way to resolve this issue is to install an M-Audio 2496 card in one of the two available PCI slots. That will give you a stereo input and a stereo output, plus two channels of digital in and out, plus a MIDI connector for in and out. Drivers work with Win 7 64 (although they don't like Skype)
Gary