UR 12 via VMWare really noisy

I just bought the UR12 interface.

When I connect it to Windows 7 via VMWare (UR12 drivers installed) it is unusably noisy and sometimes stutters.

I already tried increasing the buffer size to 2000 but it makes no difference.

The problem exists in all applications and in Windows itself (Media Player).

My integrated sound card (Mac Pro) works without any problems.

What is wrong? What could I do? Any ideas.