My 5.0L C1500 a/c works on the 2 highest blower settings at most driving speeds. When set on the two lower speeds the a/c will slowing shutdown. Placing the blower to a higher fan speed and/or turning the a/c control off/on after the shutdown has no effect. If I shut off the truck and restart it with the blower on high the a/c begins to work again. I'm wondering if this could be vacuum related.

 

no vacuum controls in that car. everything is electronic. i think you have a bad blower motor resistor block.