HEATER CORE LEAK?HEADGASKET NEEDS REPLACED?

99 FORD TAURUS LX, V6, AUTOMATIC, FRONT WHEEL DRIVE, 150,000 MILES.I SMELL ANTIFREEZE WHEN I USE THE AC.I HAD THE THERMASTAT REPLACED BECAUSE IT KEPT OVERHEATING.2 WEEKS WENT BY THEN IT OVERHEATED AGAIN. TOOK IT BACK TO THE MECHANIC. HE COULD NOT FIND ANYTHING WRONG. HE PUT STOPLEAK IN IT. HAVE NOT PICKED UP MY CAR. THE HEATER DOES NOT WORK VERY GOOD.I HAD THE AC SERVICED IN MAY.I HAD THE BLOWER MOTOR REPLACED IN JULY. THE MECHANIC THAT SERVICED THE AC PUT DYE IN IT SO HE COULD TELL WHERE THE LEAK IS. LAST WINTER I HAD THE HEATER CORE FLUSHED OUT BECAUSE THE HEATER DID NOT WORK. AFTER THAT THE HEATER WORKED ONLY WHEN THE CAR WAS MOVING.I HAD THE OVERFLOW FLUSHED OUT LAST SUMMER BECAUSE THE ANTIFREEZE WAS RUST COLORED. THE ANTIFREEZE SMELL IS NOT AS BAD NOW SINCE I HAD THE THERMASTAT REPLACED. THE PASSENGER FLOORBOARD IS NOT WET.I ALSO HAD THE TIERODS ENDS REPLACED, NEW STRUTS, A SWEIGH BAR REPLACED. NEW TIRES AND 4 WHEEL ALIGNMENT. SO FAR I HAD 4 MECHANICS WORK ON MY CAR. THE MECHANIC THAT REPLACED THE BLOWER MOTOR SAID I NEEDED AN AC KIT FOR MY AC SINCE IT WAS LEAKING ANTIFREEZE.A GUY THAT USED TO BE A MECHANIC SAID IT WAS THE HEADGASKET BECAUSE THE FORD ENGINES ARE POORLY DESIGNED. SO IS IT A SMALL HEATER CORE LEAK OR DO I NEED TO HAVE THE HEAD GASKET REPLACED?OR WHAT DO I NEED TO DO?IS IT THE VALVE INSIDE THE FIREWALL OF THE ENGINE? THE MECHANIC WHO FLUSHED OUT THE HEATERCORE SAID THE ENGINE WAS NOT BYPASSED BY THE HEATER HOSES. THE MECHANIC THAT REPLACED THE THERMASTAT ALSO PUT ANTIFREEZE IN. I DON'T THINK IT WAS LOW THE FIRST TIME IT OVERHEATED. : ?