Hi @Bruce_W,
If the LED is blinking it means that the 3.3V power supply is not working constantly. I don't know if with a multimeter we can see the behavior, probably just with a scope, but if you have a voltmeter/multimeter, please try to measure the voltages on, C208, C176 (or C177) and C178 (or C179) while you power the board from the PC with the USB cable. It will average the values, but if the power supply is that unsteady, you might be able to see some fluctuations. There's an overcurrent protection IC on the path when using USB power. I suspect that to be the cause of the problem.
Bianca