THIS MUSTANG I PICKED UP LAST MONTH IS TRICK FLOWED OUT WITH TWISTED WEDGE HEAT AND TRACK HEAT UPPER AND LOWER INTAKES. IT RUNS A CONTINUEOUS 215 DEGREES TO 220 DEGREES IN TRAFFIC OR RUNNING DOWN THE INTERSTATE. IT HAS A NEW HIGH FLOW WATER PUMP,FLUIDYNE RADIATOR, AND 160 THERMOSTAT 180 THERMOSTAT OR NO THERMOSTAT AND STILL THE SAME TEMP.
POSSIBLE INTAKE LEAK SOMEWHERE?  

Maybe you have a pinhole in your radiator?  Just a theory. Mine did.