Our Whirlpool dryer stops with code PF without a power failure. Appears to be load related as it will run cycle if less than half load. But more than half load, it stops and shows PF.

It sounds like the motor relay on the board is dropping the voltage to the motor. In any case, check all the connections at the board and make sure they are tight and that nothing is loose if the wiring checks out replace the board. If you need a part number for the board I will need your model number. Just post a new question with your model number and requesting the part number for the board.