There are two things that can cause this.

One is being low on refrigerant. The evaporator won't be cold across the entire surface and the air is not that turbulent in the air distribution housing so it can have hot and cold zones when it is low.

The other is a broken internal divider inside evaporator. There is a divider inside the evaporator on many cars (like a 740) that allows the refirgerant to enter the end of evap. and then go across the evap. turn in the other end and come back across the evap. to the exit line. If the divider in the end with both fittings becomes dislodged (it can crack) it allows the refrigerant to go through the end tank and not do its proper parallel route through evap. This again causes warm air in some vents.

To check for this you run a/c for about 10 minutes with engine at high idle speed(1500 rpm) and see if the accumulator (the aluminum can on rh side with the large a/c lines) is frosty or very heavy with condensation. If it is you likely have the divider problem, if it is cold but not approaching freezing over and not heavily sweating then it is low on refrigerant.