[Bruce Edwards]

Have you called the builder to see if he had a problem when he test ran it? I am wondering about head gaskets but I do not remember at this moment if the FE uses a only fits one way gasket. I do know if some are installed wrong the water flows out of the front and the rear has no flow.
If the builder did not have a problem then it has to be something going on since then.
I can not see how it would be a lower hose at idle. Not enough flow at low speed to cause the hose to close.
It really sounds to me like a water flow problem.