I wonder what the driving situation is for the OP, what engine rpm and what the vacuum reading might be, this is going to be constant rpm I'm guessing.

Could this be a problem with the power valve?

could he be running 5000rpm on the highway for a few miles and have the vacuum high enough that the PV starts closing?
would the motor be able to run leaner at that speed if it's a light load 5000rpm cruise?

fast68 is definitely pointing out some variables that should be looked...
I would try a known working carb to on the car.