[Oldsguy]

I have seen the heater control resistor block cause that problem. I is located in the air plenum (engine bay side) and has four wires running to it. It is mounted on the plenum with just two sheet metal screws, usually the block itself is shaped sort of like an oval. On the opposite side you will see three coils, these coils are actually RESISTORS, their length and guage determine how much resistance is offered to the blower fan. I would be willing to bet one of those coils are bad, you should see an interuptions where the wire has actually burned in two and therefore is an open to the fan circuit for the High setting. You can get a replacement from a local junkyard if your part store can't get it.