p38 diesel overheating problem

hi. help please. 96 p38 diesel. when started from cold overheats within 5 mins. no error codes given. replaced radiator, checked water pump, changed thermostat and temp sensor, been gas tested with negative results.
any ideas as to what i can do next.?
losing hair rapidly.
please help
glen

when you say it gets hot after 5 mins are you going by the temp gauge or the heat given off by the engine, also when it gets hot does the heater give out hot air or cold/warm