1993 Chrysler Grand Voyager Engine overheating

I have a 2.5 TD Voyager and some problem with overheating. I was on a longer trip and it started overheating. It was a very hot two days, 92 F. I was driving in a line of cars not much space between the cars and at a slow pass. Then I noticed that the temperature got high up, close to the point when the fan should start and it was at that point for longer time. It didnt move up or down from it. The line of cars cleared of and I started driving faster and was expecting the temperature to get down but nothing. Then when I went up the hill it started going over the mark when the fan starts working. I stopped checked if the fan is working and it was working. I waited there for some twenty minutes to cool it off while the engine on. The fan was able to cool it off but not till the mark like it was before. It stopped working just few milimeters before that mark. The rest of the way the car would coo to that mark only if I would be driving longer periods down the hill and would also, to my notice heat up much faster then usuall. Also while driving it on a highway at speed of 80 mph at night when it was cooler the gauge would show the temperature a bit above normal and even if I was to slow down it didnt cool off to the normal line.
Last year I changed the water pump, few months back I changed the coolant and I got stuck on the road cause one of the water hoses was leaking (they said the mechanic put a too strong of a coolant). That was changed and new coolant put in but few weeks after that I noticed I am loosing the coolant and could see any obvious spots where. I went to the mechanic he said it could be a broken head gasket. I consulted few other mechanics which said it might not be a problem. Country is very unstable here and it is hard to trust people, especially mechanics, plus there is no official Chrysler garage. I continued driving and drove with no problems next 4500 miles until this happened and was driving long distance on hotter days that described here. What could it be? Could it be a head gasket or something else. If it is a head gasket how can you check that? Would the can loos on power then cause mine isnt.
TX.