by your description you are either not getting enough air through the radiator while moving Or the radiator is too small / Inefficient to keep up with the heat being generated.
You can try some temporary cardboard baffling to trap and force the air through the radiator. Do not forget the seal between the hood and core support as well as any other areas the air can be escaping through. Last thought would be are fans/ shroud blocking the air flow ??? Some electric fan / shroud setups have rubber flaps that are pushed open at higher speeds letting the air flow through rather than acting as a barrier.
A long shot but make sure your lower hose isn't collapsing if it does not have a spring inside it Keep us posted