Just to answer my own question, and since nobody replied perhaps this will help someone else: The problem was a worn out blower motor. Removing the motor, which was not making any untoward noises, revealed a very stiff motor shaft. Apparently, it was pulling huge amps when it energized. That in turn caused the protection circuit in the blower motor relay/resister unit to shut off the blower as it was drawing too high a load. It would then stabilize, resume power, draw high, repeat. A new blower motor and it's all working perfectly. Easy job, too.