Mixdown freezes for 20 seconds

Since replacing the motherboard in my pc I have this strange problem
when executing an audio mixdown, it happens about 50% of the time.

I get these temporary freezes, the progress bars stops for about 10 to 20seconds,
Nuendo is responding very slow during this time (It is almost impossible to cancel the mixdown).
During this process my CPU meter in windows goes up to about 90%.
After that the audiomixdown presumes for a few seconds (CPU back to 30%) and then I get another freeze.

So a mixdown that would normaly take about a minute can now last for 15 to 20 minutes.
Then the process seems to end normaly but the result is no mixdown file what so ever.
Only way to fix it, is to restart Nuendo, then everything behaves normal… until it happens again!