You haven't said what model and year but this is fairly universal.

1. Check the coolant level. If you have a sight glass, are there
bubbles when you rev the engine? If so, there's low coolant.

2. No bubbles, then there's a vacuum leak and the car goes to the warm
side when the vacuum increases.

I think #1 is the best guess.

Go to a good AC guy and have the pressures tested. I *think* you'll
find the coolant (once called Freon) is low. Slow leaks are common and
they reveal themselves every year or two.

Also, get a thermometer for the vent (couple of bucks). You should
easily get 40 F at the vent when everything is working properly.

I used to do my own AC and readily got nearly 30 F at the vent with a
new drier, good vacuum, and coolant charge. Even with the new stuff
(134A?), I easily get under 40 F at the vent on a hot day.

Chuck Fiedler
Nothing but Volvo since 1974

In what? Could be a vacuum leak opening the heater control valve if the
car has a vac operated valve.