I have now actually successfully run Cubase 9 a few hours without audio disappearing… I don’t know what I did. But the direction is positive 
But still getting audio cracks from time to time (like every ten minutes or so). It comes randomly (i.e. even if I’m not doing anything)… I have to give this more time and see what happens and if I can elaborate more. Nevertheless, I haven’t had these issues with 8.5.
I am running Windows 10 with i7-6700K 4x4GHz and 32Gb RAM.