1998 Dodge Durango overheating May 2011

I’m not a mechanic or expert in cars what I know is very little like changing the oil or spark plugs stuff like that but it was until recently May 15, 2011 that I started having problems with my 1998 Dodge Durango of witch I own for 3 years. Well like I said May 2011 My Durango started overheating I was told by friends and mechanics that it was the thermostat, water pump, radiator  bla,bla,bla, anyway I started with the smallest thing hoping it was the cheapest so my mechanic change the thermostat put a new one, still overheating so then the water pump and the belt adjuster or whatever is called but still overheats, so I’m thinking dam It’s  gotta be the radiator so I go to ebay order one and have my mechanic installed it but dam it’s still overheating, now my mechanic decide to check the engine and the radiator with a heat gun and they both show normal temperature even when the gauge show the max 260 degrees so my mechanic thinks that the gauges are wrong and not working properly and that there is some sensor that controls the gauges and need to be replaced so again still the same, by now is May 31 2011 & my mechanic is out of options so he decides to contact the dodge auto manufacture to get answers for this problem in the meantime same day on the way  home I was spilling antifreeze the whole way home I decide to do my own research so I google it , I read similar problems many of them till 3:00am and wrote many of them for my mechanic but one of them stud out because a friend mentioned it to me but didn’t think anything of it because he’s no mechanic he said that when you haven’t change the transmission fluids for a wile or is very dry in fluids the trans tend to get hot and it causes the temperature of water to rice, please what’s that got to do with this, well anyway next day I try it change the trans fluid to my surprise nothing came out bummers I always checked it and it never need it then find out that you have to check it with the car on wow like I said I’m no expert, well I added one gallon I think is like 5 quarts and added more coolant because it spilled most of it the day before and AMEN problem solved no more overheating even with the ac on it does not pass half on the gauge all this time it just needed transmission fluid I should just shoot myself but I’m happy now just in time to make long trips for the summer.