305 overheating problems

I Have a TPI 305 in my jeep and I'm having a serious overheating problem. The problem starts after the jeep has been running at highway speeds for about 15 -20 min. Temp gets up to 245 - 250 This is what I have done so far, 2 different thermostats Down to a 180 now. New Radiator (3 Core) and...