Q:Air conditioning failure of a 1996 Holden.

Q:Air conditioning failure of a 1996 Holden Jackaroo
1. I drove my 1996 Holden Jackaroo for a long trip, outdoor temperature was about 30 to 35 degrees. After an hour or so, I decide to use air conditioning but only hot air blew out from it.
2. A professional came after the trip and could not find anything wrong of the air conditioning. He suggest it may be because I did not put air con in internal circulation and hot air from outside the car caused the problem, ie no cold air.
3. Next time when I had a long trip, I did remember to put car circulation in internal circulation mode but still no cold air coming out. Nb the temperature for that day is hotter than the first trip which was around 35 degrees.
4. Same professional came and checked the air con system thoroughly again. However, he still could not figure out what's the problem. He used gauge to check pressure inside the system which is high enough and indicated no leak of system. With car sitting in the sun on a hot day and engine running idling, also with fan of air con in full speed, ie in coldest setting, the clutch at the front of the compressor is turning and the air con system did produce cold enough air. He suggested me to try to use air con when doing long trip again and see if air compressor is engaged or not and also if two tubes connecting to evaporater(?), One supposed to be cold and the other one supposed to be hot but not burning hot, are in normal condition of temperature or not.
5. This time tubes with burning hot temperature and clutch at the front of the compressor not turning. It was an even hotter day. It happened when engine was idling and air con was on. That is, I sat inside the car and was enjoying cold air. Nb car had been driving for about an hour before stopping and idling which means engine has been very hot already. After about 15 minutes air con stopped working and start to blow hot air. Also, coolant of engine's cooling system started to boil and leak to ground because of the very high temperature from engine. It looks to me air compressor stopping working because of too high temperature in order to protect compressor. I stopped engine and wait for about half hour to let engine cool down a bit. I could drive my car since then without using air con for the rest of the trip, ie another one and half day. Actually, I used the car for some short trips, around 10 minutes, after coming back from long trip and air con worked fine again.
6. I would like to have some more understanding of the problem of the air con of my car before getting back to the same professional again. That professional said the burning hot tubes of air con system indicated high pressure in the system and caused failure of air con system. My question: what cause high pressure in the system and how to fix it?
7. The other question is what cause high temperature of either air con or engine system or both when engine and air con both are running for longer time, e.G. Over one hour? Does the symptom indicates cooling system such as radiator failure or partly blocked cooling system? And how to fix this problem? Thank you very much.
8. Overall, it looks to me air con system of my car seems working fine and it is engine's cooling system having problem. Am I right?