Since: Jan 18, 2003
i don't know too much about this, but it might be a buffer size thing. someone else in another thread got rid of a similar issue by enlarging the soundcard's buffer size but the tradeoff is greater latency.
is it a clipping problem? if your mixes are like mine, they mix down a little bit less in volume than what they are in the app, so make sure you're not clipping while monitoring. that sort of problem might vanish upon export.
put the soundcard control panel up while monitoring and see if it's peaking in the red. if it is, you have to reduce the output signal from cubase. if that's not the issue, check the buffer size.