I have a 94 Spirit that does that. It has a small leak in the heater core that seems to have plugged itself (I did not put stop leak in it, but it is an old car so I don't doubt that it has some gunk floating around in the cooling system). I ran with the cap loose for a long time because otherwise it would pee on the floor and smell bad. The leak, while it doesn't seem to be actively leaking out into the vehicle, now, with the cap half-latched, appears to be drawing air into the system when it cools down, which puts a big air bubble right in my heater core, and it expells coolant through the cap when heated up now, which then requires the addition of more coolant.
Long story short: don't rule out a leak in the heater core that you may not see yet (not sure if the coolant would leak into the floorboard or if on the intrepid it would instead come out through the AC drain tube). If it can leak fluid it can leak air and it will have a tendency to draw in on a cooldown.