try these setting with your Fireface and Cubase 8, I have tested these with my system specs and a fireface 400 and it is good to go:
RME fireface settings:
WDM devices = 0
Enable MMCSS fo ASIO = Enabled
Cubase:
leave ‘Activate steinberg power scheme’ unchecked (disabled)
ASIO guard = high (hopefully clicks and pops are gone for you at mid settings as well with fireface at 256 samples)
Windows OS:
in your windows power options go to advanced settings:
USB settings:
USB slective suspend settings = disabled
intel graphics settings:
use maximum performance
PCI Express:
Link State power managment= Off
Processor power management:
Processor performance core parking min cores = 100%
minimum processor state= 100%
maximum processor state= 100%