Switching problem from heat to air

First of all you need to find out the actual problem, it could be a servo motor that is getting weak and can't pull of push the door open or closed. On the other hand it could be a control module or a simple switch that doesn't catch. The most cost effective way is to have the unit checked by a good independent shop,( it's old enough that you don't need the dealer) they can let you know what it is going to take to fix the problem.