Any ideas about what would cause the following : Heat was working fine until last Wednesday evening -- I always just left the control on "Auto" and the setting on "economy" and only slightly adjusted the 'temp control wheel' when necessary. And of course use the "Defrost" setting when necessary.

Now the heat comes from all the vents 'wide open' no matter what the setting is on. Sometimes the fan will slow down, and this seems related to engine speed (vacum). In fact, on rare occasions, the heat seems to work fine. The real problem is that there is either 'too much' heat, or when I turn the thing 'off', (which the system is somewhat slow to do), there is no heat. Any suggestions ? If I shut off the side vents and center vent, heat will pour out the defrost ducts and overheat the car interior even though it is only 15 or 20 degrees outside. The floor vents do not seem to be working proplerly.