I run the 160 quick opening high flow Thermostat. Stock Cast iron water pump housing and cast iron water pump drive. 26 inch 3 core stock rad and the stock shroud and a stock seven blade fan without a clutch drive. The motor is a warmed over 430 inch Hemi all iron hemi. Mechanical gauge is taken of the water pump and shows 163 degrees idling, driving or anytime. Maybe you are not holding the water in the tank long enough to cool it, if the capacity is too small. Maybe your water pump and housing are not mated if you have had this same problems with other rads without other changes. Now was you block cleaned before rebuild in the cooling passages.