Does this have a center tap on the battery? meaning a 3rd cable in one of the center bars. These older trucks ran the Aux functions at 1/2 battery voltage usually 18v to get the slower speeds. If the connection of the center tap at the lead bar is corroded or loose it will act like this. You will need to tighten this connection or replace/ add an insert to the lead. The contactors in back will flip/flop each time a aux function is called for, using the positive side of the battery then next request will pull from the negative side of the battery.