Join the club my friend..your just another customer who has (like me) fallen into the trap of paying to be a beta tester,
I have so many issues with CP8 that I have just stopped using it, (only just got it to open properly) I think this will run the same as C7...ie it will never get fixed properly and then they will release CP9 which ''will fix everything'' and so the merry go round will continue...
for some, but not for me...
it's not my system because C7.5 runs solid.
I completely understand the bafflement and frustration if something like this happens. However, with no intention of trying to be provocative, I really don't think you can say it is nothing to do with your system. My system in terms of hardware is fairly similar to yours: I have a 2600k slightly overclocked and with just 8Gb ram. Cubase 7.5 runs fine on my system; so does Cubase 8 Pro. There are so many possible conflicts/interrelationships with software and hardware. To progress your hypothesis that 'it's not my system' you/we would need to do more tests and get more information from fellow users.
I take the view that we all need to live with uncertainty when it comes to wanting to have a definitive conclusion about what is causing a problem in our particular hardware/software universe (which is significantly diverse from one user to another). That's not to say there aren't problems; I'm just suggesting we all need to be cautious when proposing what is causing the problem or, in fact, what we assert is definitely not causing a problem.
Hope you manage to straighten this out.