Is the sensor plate properly centered and clean, such that it does not bind or stick in the "down" position?

High fuel pressure can cause this problem because it raises the control pressure. This control pressure resists the lifting of the sensor plate -- higher cotrol pressure means more resistance. Two cause of high fuel presure are (1) a blocked fuel return line, and (2) a defective or plugged fuel pressure regulator.

The regulator is part of the fuel distributor. If you changed the fuel distributor with the same results, then it's unlikely that the fuel pressure regulator's at fault.

The control pressure regulator is a candidate for suspicion. They can be disassembled and cleaned, but before doing this, I'd get the proper fuel pressure gauge set (which attaches at the CPR) and monitor the pump pressure, rest pressure, and control pressure. If the control pressure is causing this, the pressure measurements will immediately reveal it.
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Don Foster (near Cape Cod, MA)