I finnally stopped mine from switching. I added a vacuum tank, NAPA part No BK7301605.
I had read about phantom_smack solving his problem the same way. ( Vents switching under engine load and losing vacuum because of a failed check valve.)
Tank cost about $13 and is about 6 inches in diameter, plastic with two nozzles and an internal check valve.( If you draw on the nozzle, without the hood, you will be able to draw air---but you will not be able blow through. This is the nozzle to connect to the engine side.
I removed the inspection cover on top of the dash to watch the valves move. There is a1/8 inch black rubber vaccum line, on the left side of the engine coming out of a block with other lines. There is a connector in the line. First, with the engine idling, I operated the slide control to turn on the air conditioning, and watch the valves move in the dash. Then I pulled the vacuum line loose and operated the slide control again, and when the valve did not move---it was the correct line.
I used another piece of 1/8 inch tubing, about 6" long to connect the tank to the line from the block . Drilled a small hole through the tank mounting bracket and tied the tank under the hood with a piece of wire. Connected the line going to the dash on the other tank nozzle. | 
