Sounds like a blockage. You said that you flushed the radiator, did you flush the motor at the same time? Or just the radiator. Either way there might be some solid blockage in the core some where best I think to take it to a radiator shop and ask them to clean out both the radiator and the motor (they can do it with a much higher pressure than you can with a garden hose, which I assume is what you used).
