ECM Blower Motor Not Working at Night
Since all signs seemed to point to a bad motor, I took a chance and replaced it.  The new motor has been working fine for several days now.  Not exactly sure why it was only failing at night, but I still think it is related to the slightly higher voltage fluctuations I get at night.  I did power the old motor directly for high and low voltage inputs (removing the control board from the equation) and experienced the same problem when the voltage moved higher and then it was fine when the voltage came back down.  Not sure why.  Could a bad thermistor cause this?  Not sure.  Unfortunately the circuit board in the control module of the motor is potted and I don't see how I could fix it.  Too bad.  The motor was only 5 years old and it would be great to repair if possible.  Anyway, I hope this helps anyone else who may experience similar problems.  Thanks all.