Hi Tex!!!
That happens when the motor have to work with too much load, not necessarily the propeller load, could be that something else, like a bad alignment of the motor, but you solved it with the dog bone, another thing could be the shaft or the tube that contains the shaft is not straight.
My advise, remove the coupling system and turn the propeller with your fingers, if it moves easily, then install the coupling sockets and check that they are not grazing against anything, then install the hole coupling system ant turn again the propeller with your fingers to notice if there is not too much difference in load with the hole system assembled compared with only the shaft and the propeller
Best regards