I bought Cubase a little over a week ago, but was unable to get it to work correctly until tonight. 
 
The audio would drop out even with only a single audio loop playing, even at the highest latency settings.
 
It turns out that Avast antivirus does not play nicely with Cubase, at least on my computer (Creation Station CS450v3). 
 
I uninstalled Avast and have started using Windows Defender. So far, so good.

Antivirus software in general plays havoc with DAW software, as far as I understand.

I never encountered problems using Sonar with Avast, nor do I have problems using Cubase with Windows Defender. Knock wood.
 
But yes, I have heard of others having antivirus issues.

I am using Norton Antivirus. It leaves my Studio One, Sonar and Reason 10 in peace on 2 machines.