By the photo you have it looks like that whole setup is acting like one big winter front: it's completely blocked off. That may work for the high-efficiency rad in the car those fans came out of, but it's not working with your setup.

Cut one of those fans out of there and remove half the shroud that goes with it. No wonder it's heating up on the highway: you have no air flow. Explains why you have to stop and the fans can cool it while sitting still.



If you don't want to do that, at least get the hole-saw out and cut some daylight in that shroud between the fans to open up some air flow.