Climate control settings,on all vehicles

you say it will blow cold and hot air for only a few minutes. You don't say but I assume you mean will blow cold a few min. when turned on AC max , blow hot only a few min. when turned to heater hot. Assuming thats the situation and you say AC charged and shows good on the low side -you should charge the AC to a full charge on the low side will cause problems with the AC air compressor kicking on and off. I suggest while the engine is running and turned to AC cold max watch the AC air compressor clutch to see if it does indeed kick on and off if so charge completely. As far as the heater make certain you coolant is all the way to the top and the best way to do this is when engine is completely cold remove radiator cap and fill completely and completely close cap then fill overflow container just to the cold level. That should fix your problem.

When you say you checked the pressure did you connect gauges to the high and low sides to get both pressures, this sounds like it may be low. also what is the temp coming out of the vents. note on really hot days and humid the ac system doesnt cool as good unless the truck is moving down the road or highway and does take a while to cool down. at a stop or sitting or in traffic the ac temp will rise as the condensor needs to be cooled like the engine does, if its not cooling then the temp will not be cold, check the front of condensor for blockage, also make sure the fan is running with ac on. this fan should run when ac compressor is on,maybe clean condensor with water hose blowing from radiator outwardto front bumper, then make sure the hot/cold blend door is closing all the way,a quick test is to turn a/c on, turn temp control to hot and see if it gets hotter then back to cold and it should get colder. this sounds like the system may just be a little low. have an a/c tech check it, you may find its low or nothing is wrong. good day.

this may be due to a few things, it may be a bad blend door actuator, this is the motor that turns the door to give hot or cold air, the next thing is maybe compressor connection is loose or the system is low on gas. start the car and see if the compressor clutch kicks in then if it does try turning the temp control to hot then cold and see if that works maybe the blend door is stuck,

I would check the following possibilities:
1. Is compressor clutch working? - You need to do this when the ac is blowing hot air only - locate compressor drive pulley on engine - rubber belt wraps around it. Start engine and set to max ac. When the ac is blowing hot air, look at the center of the compressor drive pulley using a good flash light - watch it for a while, it should be spinning with the pulley - this means the compressor is engaged. If not, the compressor is not engaged. If your ac is blowing hot air, then the compressor should be engaged and running (trying to cool). If it is not engaged, then your compressor clutch is probably going out. Could be low voltage to clutch, but more likely the clutch is worn out and only working some of the time. Note: if working normally, the compressor clutch will engage and disengage in cycles- but the air should always be cold. If clutch is bad, compressor would need to be replaced (sold as one unit). This is tricky to diagnose and could take some time - you need to do it when ac is blowing hot air but set to max cooling.

2. Low on refrigerant or refrigerant has moisture in it? If your ac blows cold initially after being off for a while, then heats up, and continues to regularly cycle back and forth between hot and cool air, then the system may be low on refrigerant or the refrigerant may have moisture in it that is freezing and restricting refrigerant flow. An a/c shop can check refrigerant level and even replace refrigerant if moisture is suspected.

Could be other issues, but I would start with these.

Good luck

AC will not work with temps are cold even when it is turned on. The air is colder cause the temp is colder. The AC cycles in hot weather because the AC charge is low and needs recharging. You will know this if you open the hood run the AC and you will hear the clicking sound of the AC compressor magnetic clutch. You can recharge it easily buy buying a compatible can of refrigerant. I suggest buying the kit with the gage so you don't overcharge the system which will cause freeze ups