engine overheats

I have a Transit 54 2.4 125 and I am experiencing some problems with it basically about a month ago I carried a vehicle on the back of my recovery truck was not too heavy but the truck has not been right since. basically the problem I am having is when ever I take it up to higher speed or attempt to go up a hill the temperature gauge was rising into the red and losing power felt like the turbo was not boosting enough, since then I have had the van into the garage numerous times to see if we can cure the problem we found that I had a slight leak on the radiator so was replaced. i also changed the following parts the water pump, temperature switch,the
thermostat, turbo, injectors, viscous fan coupling, but all to no avail. The temperature gauge is not as bad as it was rises up to the three quarters then soon as you accelerate rapidly goes back down to normal. EGR valve has also been cleaned out. mechanic tested the pressure of the shaft and held liquid. also been scanned for codes and nothing comes up. I am a bit lost at what is causing the problem as I have spent over $800.00 trying to fix it, but having no luck and I do not want to just guess what the problem might be as it is starting to add up now. lol. i am just wondering if anyone has any ideas on what it could be? I do not want to break my van and at the same time i need to be able to rely on it as I would not want it to break down with a customers car on the back. any help would be much appreciated.