95 regal - heater control is bad?

1995 buick regal: When I turn the heat OFF, hot air will push through the heater vent (by my right foot) when I'm driving on the highway, then it stops when I come to a stop. Clearly outside air is pushing in, gets heated by the heater core, then pushes into the cabin through the heater vent. When I turn the AC on, then the air stops coming through that heater vent, and cold air starts blowing through the front vents. When I turn the heat OFF, and slide the temp control to cold, then cold air starts pushing in the heat vent, when I slide the temp control to warm, then hot air pushes into the vent.
Turn on the AC: no problem. Click the OFF button: heat flows into the car. My right foot feels like it's on fire, I can't keep my foot on the gas for longer than 5 minutes, it would be nice to drive around with the windows down.
A mechanic on justask_com says this is by design, but I don't buy it, it was not doing this a year ago.
Any ideas which parts should I replace?

I would first check the vacuum lines and the canister that supply the HVAC system. On your car, all the actuators except the temp control are vacuum actuated. When you turn the HVAC control to off, this shuts off the flow of engine vacuum to the actuators. If there is a leak, since the fresh air inlet door is vacuum, it may be bleeding off and opening when it should be closed. This allows outside air to flow through the system, allowing air to seek a path of least resistance which may be through the heater core to your right foot.

:newburn: