a hpco (high pressure cutout) switch senses high pressure and opens the low voltage circuit controlling the compressor contactor. the outdoor fan is probably on the same circuit with the compressor and they will shut down simultaneously. with the compressor off, the two operating pressures begin to equalize and the pressure drops below the setpoint the hpco switch is calibrated for. this actually happens within a minute of shutting down the contactor. after that, a reset is required to energize the contactor again. the purpose of the hpco switch is to protect the compressor from running under excessive pressure. in heat mode, factors that affect discharge pressure have more to do with the conditions in the indoor airflow and heat transfer in the indoor coil. the indoor coil and outdoor coil reverse roles in heat mode, so the problem is poor heat rejection inside due to the air flow issue or some other reason for poor heat transfer. the surplus heat that has not been transferred inside recirculates and drives the discharge pressure up, causing the pressure switch to trip. to determine if it is an airflow issue, static pressure can be read across the system inside.
your technician said the charge is ok. if the problem was solid, a pressure imbalance would present itself. if it is being checked early in the day, maybe it doesn't present due to lower temperatures. he said airflow was probably causing the switch to trip, but there's no mention of data gathered that would support a theory. sub-cooling and superheat are common calculations we use in the field and if there is a heat transfer problem, those numbers would be off.
if i am off base in my statements, someone here will set me straight i'm sure. i doubt if the supply ducts are the problem, but the return air opening sounds like it could be too small. i'm assuming you are 100 % sure there are not filters in two locations. by the way, what size is the return air opening in the house ? also, warmer temp inside tends to bring on the problem. that's a clue also.
one more possibility is a partial restriction in the liquid line feeding the outdoor coil. this could possibly be a partially restricted filter/drier, or a partially restricting metering device. when that happens, discharge pressure could potentially rise.
so far no one has actually pinpointed your problem. if it is intermitent, no one wants to baby sit the unit to wait for a problem to surface. without knowing operating conditions and readings, there is no way for us to help you. even if you replace it, you need to have your duct system evaluated. it must be frustrating to have so many people out and not get a conclusive answer. we are making educated guesses, but data should be gathered and analyzed. you know what they say about beating the dead horse.