The high speed function of the blower on my vintage air unit works fine for the first few minutes after start up, and then starts blowing on the medium setting. I called V A tech today and they told me it was probably a faulty switch. Granted, the switch is the original 40 year old one, and it could be faulty. He also said that there is a diode in the high speed circuit; if the diode went bad, there would be no high speed at all because diodes don't overheat, they just fail. I was wondering what opinions are out there. The only other option is a failed heater motor. If that is the case I am going to be very unhappy because you have to remove the unit to change the motor.