THE CONVERTER WAS CHERRY RED NOW IT WON'T START AT ALL ?????????/

HAVE A 1988 BUICK REAGLE 2.8 MOTOR 45K MILES I DRIVE THE CAR AND IT IS KIND OF SLUGISH BUT I GIVE IT A LITTLE GAS AND IT GOES BUT WHEN I COME TO STOP SOMETIMES IT STALLS THEN IT WAS STARTING A LITTLE HARD I DROVE IT TO THE STORE AND WHEN I CAME OUT I BARELY GOT IT STARTED WHEN I GOT IT STARTED IT WOULD ONLY DO ABOUT 20 MPH I HAD NO POWER LOOKED UNDER THE CAR THE CONVERTER WAS CHERRY RED SO I CHANGED IT I THOUGHT IT WAS PLUGGED NOW WHEN I TURN THE CAR OVER IT WONT START ????????? JUST FIRES AND POPS LIKE ITS FLOODED OR SOMETHING SOMETIMES IT ALMOST STARTS RUNS ON 1 CYLINDER OR SOMETHING IT LASTS FOR LIKE 15 SEC'S . THEN DIES ?? CAN YOU PLEASE HELP ME FIGURE OUT WHAT IS DOING THIS ASAP ??PLEASE & THANK YOU