You will never be able to "flush the radiator" correctly. Take it to a radiator shop for $70 they remove the upper and lower tanks and put the core in a hot bath and clean the tubes out. More than likely you have set-up for a cracked head. None of mine overflow out of the bottle. It is normal for just a little to go back and forth but not enough to boil out of the canister. The gages on these things are bullet proof. Replace the temp sensor located on the front left side of the head next to glow plugs or check the harness connecting to it (Part of the injection pump harness). Now if you have a rusty truck I might believe you the gage is done but otherwise they last forever. Let me know if you need help as I have parts to share if needed. The pointer should never go more the 1/4 way up on these. Mine always runs about 210. 230 is very hot. The death of these motors is heat so fix it before driving more. Hint if you want to try a new radiator just find one for a 4x4 2.6 as they are the same size just have the cooler for the auto trans. MB221988, MB222043 were factory part numbers. Start googling to find the aftermarket replacement Hard to find but they exist. Basically the stock radiator worked perfect when clean. Just a little plugging up and they over heated. Radiator pros makes a good radiator with better cooling.

Mike