It's also possible that the system is not cycling off properly and it's turning the evaporator core in the dash in to a large block of ice which won't allow airflow properly.
You really need a set of gauges on the system to tell best but you could at least put a digital thermometer in the center vent and turn the a/c to recirculate (Max A/C), temperature to cold and the blower speed to the lowest setting.
This will cause the compressor to cycle off and on the most as well as get the evap core as cold as possible. From there you can check the operation best.
Situations like what you're describing can be caused by a bad pressure switch, evap freezing up, compressor clutch gap being excessive and the magnet eventually can't pull it in, or a random issue such as wiring.
If it gets as cold as desired (or better) then you can almost rule out the charge level. Sitting still, if it's overcharged then your system should shut down within a matter of minutes since the pressure climbs pretty quickly. If it's low then your compressor would cycle off and on rapidly AND you wouldn't have the cold temperature at all.
Doug