You should check for air in your pump or something that has clogged due the coolant.
Already done - A complete strip-down clean and check of D5 pump (which was brand new), CPU block (also brand new), GPU block and reservoir, as well as flushing of all components in-situ including radiators, followed by a rebuild using brand new tubing.
Everything was as pristine as possible. The MPS only went in afterward and the system fully re-bled.
So unless Mayhems X1 can go from brand new and perfect, to clogging systems in less than 12 hours, the loop is completely free of air and obstructions. He's taking the MPS out this weekend, so we'll see how the loop performs post-meterectomy...