80 criser cooling

Hi All, On a recent trip from Qld to Vic towing a camper trailer I had sever heating problems. However I was able to get from Goulburn to Melborne where I replaced the water pump and put in a new radiator (found out the rad was stffed when I pulled it out to get to the water pump). Now the old girl will run fine except for when I first start out. She gets very hot but when I stop on the side of the road the temp comes back down and will then stay at 1/2 way on the gauge for the rest of the day, no matter how hard I push it. The C.T weights in around the 1.1 ton mark fully loaded and I only ever experience the heating problems when I was towing. before this trip she ran at the 1/2 way mark all day no matter how hard I was pushing it either on road or off. Since replacing these parts I get a heating problem no matter if I am towing or not. The only things that wasn't replaced new in the cooling system is the thermostat and rad.cap. With the diffecultly in getting to the termostat I was after some opions before I start pulling things apart. My 80 series runs a 4.2 with DTS turbo and intercooler running 16 psi boost. Also I have heard from different sorces that the 80 series do seem to have problems with the gauges being accurate, do people know if this is true or not.