Originally Posted by LoosBenz

I'm not a thermodynamics physicist, but in my mind if there isn't much difference between the temp across the radiator, it would seem like more of an air-flow problem than a liquid flow problem. If the coolant was flowing slowly through the radiator, there would be more time for heat transfer and would seems like temp differences would be relatively large.
I'm thinking it may have to do more with your shroud/fan setup that may actually be hindering air flow at higher speeds. Hope this helps.