I know this may sound stupid ... did you checked the sleep options?
Through power options? Yes, I did, should run forever.
Adding system RAM will probably not solve your problem unless you notice that you are running rather close to your current 4GB.
Have you tested both of your PSUs individually? For example, connect only one to the rig, and attempt to mine with one 1080ti and 2x 1070, then repeat for the other PSU.
Thanks for the tip, will try doing that now.