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
Yes I quoted myself as i wanted to say I have spent a reasonable amount of time in the last 20+ years resolving the exact issue(s) you are describing. In addition another 20 or so years of temperature measurement in various R & D positions.
I did leave out a couple of critical facts. When approaching a heating problem the very first thing I do is install a Thermocouple probe and digital meter to establish exactly what is going on and when. The two items can be purchased rather inexpensively ( under 100.00) if one shops for pricing. Do be a ware that there are different materials used in the probes and the are not interchangeable. So if you have a meter calibrated for "J" type probes you have to make sure the probes and extension leads are of the same type.
I do not trust gauges or IR guns.as i have seen them both be off by 40 degrees or more ( usually at the warm end)
Your problem IMO lies somewhere in my initial comment. It may also be combination of airflow and radiator efficiency. I will also say that I seriously doubt a water pump is going to resolve the issue unless your pulley ratios are skewed and slowing the pump down to much at higher RPM's.
