Knowing that we can be certain that beyond a relay failure (very rare in this particular fashion.... though easy enough to rule out by swapping with an adjacent one) the problem you are experiencing is a safety shut down. We know this because the AC ecu has turned the relay off, requesting the AC to stop operating.
The AC ecu will shut down the AC if the pressure is out of range (charge level problem) if the pressure is perceived out of range (pressure sensor problem), if the fan speed is not detected (not the issue here since your problem comes and goes), if the evaporator core temperature is out of range, or if the AC ecu itself malfunctions.
Since you have recharged the system (Assuming by machine and not "top up" cans) we can rule out a charge level issue. If it was not charged properly and was just topped up.... then a full evac and scaled charge is in order.
Since the pressure sensor has been replaced we can pretty safely rule it out too.
The fan speed detection we can rule out since your problem only happens on hot days etc.... the nature of that failure is wiring related or fan inoperative in the car, which wouldn't fit your case.
That leaves us with the evaporator temperature freezing or a problem with the ecu as your most likely suspects. With all the above correct and ruled out, what I would be doing is reading the thermistor value on a scan tool during normal operation and once it is malfunctioning to see what is going on there. What we often see are the thermistors falling out of position or getting reinstalled improperly in the past etc leaving them in a position that reads colder than expected and you'll have an abnormally cold reading cause a shut down.
The second thing I would consider is if the evaporator is literally icing over.... this can happen if the charge level is just a hair off (~5-10% undercharge) or if the drain is blocked and water is building up, as the water will start freezing faster than it can drain out.
If the thermistor value is normal and we already know the sensor is good and charge level is spot on but it is still shutting down the AC (relay power is cut), then and only then would I suspect a bad AC ecu (control head).