1999 Mercury Villager CATIALITIC CONVERTER OVER HEATIED GLO

Engine Performance problem
1999 Mercury Villager 4 cyl Front Wheel Drive Automatic 136k miles

MY MECHANIC HAS GIVEN UP TRYING TO FIX PROBLEM
SHORTLY AFTER FILLING CAR UP AT SHELL (NITROGYN) ENRICHED GAS ON AN EMPTY TANK THE CAR BEGAN TO BOG DOWN EVIN BACK FIRE AFTER
RUNNING FOR A SHORT PERIOD OF TIME.
MECHANIC TEST DROVE SEVERAL TIMES SO WE PICKED UP AND IT BEGAN TO BOG DOWN AND BACKFIRE. DROVE IT 1/4 MILE IN THAT CONDITION AND SMOKE STARTED ENTERING CABIN. LOOKED UNDERNEATH AND THE CATALITIC CONVERTER WAS GLOWING RED. REPORTED THIS TO MECHANIC AND HE WAS ABLE TO DUPLICATE THIS CONDITION AGAIN WHILE RUNNING COMPUTER TEST BUT COULD NOT FIND ANYTHING WRONG EXCEPT THE CATALITIC CONVERTER WAS CLOGGED FROM BEING FUSED BY THE HEAT.
HE REPLACED GAS AND STILL THE SAME.