CREW HAD INFLT TURN BACK WITH EQUIP COOLING OVERHEAT WARNING.

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EMER LNDG. JUST PRIOR TO DEP AT DFW; WE HAD AN EQUIP COOLING OVERHEAT LIGHT AND EICAS MESSAGE. WE HAD EARLIER RETURNED TO THE GATE WITH THIS SAME PROB. MAINT CHKED OUT THE SYS. CONFERRED WITH MAINT AND CONCLUDED THE CAUSE WAS MOST LIKELY THE EXCESSIVE RAMP HEAT. THEY BELIEVED ONCE WE WERE AIRBORNE AND BETTER AIR FLOW AVAILABLE; THE SIT WOULD BE NORMAL. WITH THAT IN MIND; WE SWITCHED TO STANDBY AND DEPARTED. ABOUT 5 MINS AFTER AIRBORNE; THE OVERHEAT LIGHT AND EICAS MESSAGE REAPPEARED. I MADE THE DECISION WE WOULD NOT MAKE THE FLT OVERSEAS. SINCE THERE WAS A POTENTIAL OVERHEAT SIT; WE DECIDED TO RETURN TO DFW ASAP. RATHER THAN TAKING AN ATC VECTORING DELAY; I DECLARED AN EMER AND ASKED FOR AN IMMEDIATE RETURN TO DFW. WE MADE AN UNEVENTFUL OVERWT LNDG ON RWY 17R. THE EMER EQUIP MET US AND FOLLOWED US TO THE TERMINAL. UPON ARR AT THE GATE; IT SEEMED THAT THE COCKPIT WAS TOO WARM AND THE SURFACES WERE WARM TO THE TOUCH. I WAS INFORMED THAT A BRIEFCASE UNDER SEAT WAS HOT TO THE TOUCH. I ASKED PAX AND CREW TO DEPLANE LEAVING ALL PERSONAL BELONGINGS. WHEN WE LEFT THE AREA; MAINT WAS INVESTIGATING THE PROB.