I was at a stop light and change the air flow from the heater from the floor to the defroster. The RPM's started to drop and the car felt like it was going to stall. I returned the air flow toward the floor and the problem was gone.

I experimented with it and found that every time I set the air flow toward the defroster, the car felt like it was going to stall. The other settings were ok.

Any ideas what could be causing the problem?

My bet would be on a vacuum leak in the dash either the HVAC control head has an internal leak, the vacuum line to the mode actuator is broken, or the mode actuator itself is faulty. Looks like you need to open up the dash and see what's going on inside.

Do you think this will be a difficult project. A while ago I heard that replacing the actuator was quite an effort. I don't want to get into to this and find I would have been better off taking to the shop.

Do you have auto climate control or manual?

Manual