transmission overheat F-350
 

On a very hot day in July on I-95 I was running 75mph on cruise, had a/c on and been running that way for over 6 hours and not towing when indicator light on gear shift began flashing. Had to pull off road, Checked that no fluid on ground under truck, put truck thru gears and ck'd trans fluid, all ok. About 10 mins later cut a/c off and continued on way. Next day went to ford service had trans flushed to include torque conv and let them do a diagnostic. Told me trans had overheated and wanted to install new radiator and trans cooler for 1,200.00. Engine did not run hot and I have 2 coolers, one is a Hayden 18,000 BTU. Went to another ford service and he said 1st dealer full of it. I have trans flushed every 35,000 miles and radiator last flushed at 50,000. Truck did this once before in WY at high altitude, again not towing. Does anyone have an idea. Thanks.