Here`s an idea, but maybe not a good one, take out thermostat, take off top hose, (engine cold), start engine if pump is good, water should shoot out everywhere. Only need to run for 30secs to check. As Datatek says it sounds like a waterpump failure, although you say it`s new. Relatively easy quick check to eliminate this possibility?
Also as said before on this thread, (sorry can`t remember who) there could be a manufacturing fault in a new rad. As a way to check this, maybe, take off both hoses, fill with hot water from top, and check it`s warming evenly as the warm water flows thro??
I`ve not tried this myself, just throwing ideas out for comment.