I’m having the exact same problem here as what the OP described…the longer I work, the worse these spikes and dropouts get. Finally after a time, I have to shut down Cubase and reopen it and then it’s fine for a while. The spikes eventually become constant, every couple seconds or so even with nothing playing back. Cubase can just be sitting there not doing anything and I can watch the ASIO meter bouncing up and down and peaking out every couple seconds.
I rarely post here, as I’m usually working happily away and don’t have any need to post, but this is driving me nuts, was never a problem with previous versions. I do have an NVidia GTX 760, others posted that they suspect that the graphics card is to blame, but I’ve been using Nvidia cards for years with no problems until upgrading to Cubase 8.
Can anyone else confirm if this has been a problem for them as well?