About a year ago, I had extensive work done on my '75 280's heater. The problem, at the time, was related to antifreeze appearing on the passenger side floorboard. That problem was corrected. Since then, I've had a continuing intermitent problem with the front panel controls. 50% of the time, whenever you select defrost or any other control setting, the air arrives at the correct outlet and at the selected temp. The other 50% of the time, and this can be the first time you activate the controls or just an abrupt change while it is operating perfectly fine, it just blows cold air out the front vents that are for the AC only. I've since had the car to a radiator/AC/heater specialist and they traced all the vaccum hoses to be sure all are connected. So, unless there is a hidden problem with vaccum hoses, that is not the problem. I've had the problem looked at by two different repair shops, and none has been able to keep it working 100% of the time. Anyone else had same problem? Any suggestions would be appreciated. Thanks.