A torn diaphragm on the mono-heat valve will do exactly what you describe. If its a European car with dual temperature wheels it has a heater valve with two inserts.The heater valve should be behind the false firewall to the right of the battery(looking from under the hood). It has 4 slot-head screws in the top that hold the insert in the housing. The insert costs around $30 and is easy to replace.
Regards,
Randy D.