Problem w/ 1998 Buick regal not starting

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