Your T.I.T. indication is low and FUEL FLOW is HIGH. So, you have two sets of 9 thermocouples (18 total); one set for the TIT indicator and one set for the Temperature Datum system. In the flight range (above 65 degrees throttle travel) with the T.D. control switches in AUTO the T.D. system will give or take fuel to match THROTTLE position to a preset TIT temperature. The fuel flow data is not used by the T.D. sys! So, if there's damaged thermocouples your T.I.T. indication is wrong and you should ALWAYS assume the T.I.T. input to the T.D. sys is wrong too. With damaged thermocouples the FUEL FLOW is more correct for measuring engine power.
You want to play it safe especially while flying by taking the T.D. system's TEMP CONTROLLING feature out of the picture by switching to NULL because the T.D. sys only knows what the thermocouples are telling it. In NULL the T.D. sys is no longer trying to match the T.I.T. to the throttle position (above 65 degrees). If you have damaged thermocouples then your T.D. sys has wrong information! You don't know the real problem until you get on the ground. You have bad thermocouple(s) AND your engine could be damaged (F.O.D., damage from overheat, incorrect fuel nozzle spray pattern or fuel control problem) and you only want to cautiously troubleshoot this problem on the GROUND.
Your EP's state fly in NULL so that the T.D. is no longer trying to match T.I.T. data. Pushing your T.I.T. to 910 degree for a temp controlling check is not a good idea because the T.IT. is higher (maybe much higher) than the indicator shows. Don't troubleshoot this problem in flight because you will most likely further overheat and damage the turbine!