It is temperature control. Your car is wanting to breath or create a constant flow of air and will adjust itself with heat/cool or outside air. You should have an option to go manual and override its desire.
Check the cabin filter if applicable.
it could be the heater is leaking!
My ac blows fine through the overhead, floor and side vents but intermittentely throught the front vents. What might be the cause?
The motor which blows air through the vents.
It should blow through the floor vents. Modern vehicles generally use movable baffles in there HVAC systems, to direct air flow where the controls indicate. If the controls allow airflow to the floor, but nothing flows out, there is a malfunction between the control switch and the actuator that directs the flow.
Grand Marquis are designed and wired so that the VAC door fails and will even cause engine not to start due to wires be connected together under dash.
middle of the dashboard, above the center vents.
a person satnding in the way of the vents
I'm not sure what vents you are talking about, but usually air vents are known to be working if there is air flowing through them.
lava
The thermostat may need to be replaced.
I found out what the switch is. It osculates the two air vents it sits between.