No high speed fan

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.