You should try to unload as many non-needed processes as possible. Have you tried a sample rate of 48 kHz? strangely on some cards it works better than 44,1kHz combined with ASIO-drivers.
Ean is still right, it looks either like a driver issue or one with your configuration (but I'm on Windows) so I can't give good advices.
Have you checked the NI forums for other users with similiar problems?
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