I think you have a refrigerant leak and the system is low. This can cause the evaporator to ice up, which restricts air flow. When this happens, take off the access panel and check. I think the fan motor will be happily churning along...my guess the evaporator is covered in ice, blocking the air. The evaporator coils are the part that is in the air duct - usually on top of the furnace; if you have an air handler, it is inside of that. In my limited experience, when they start to leak, unless you can find a fitting that has a problem where you can fix it easily, it may end up being time to think about replacement, since it is probably the compressor or the evaporator coil.