2004 Hyundai Sonata Question: What is wrong with my hyundai? Could it be my fuel injector?

DECEMBER OF LAST YEAR I KNOTICED MY RPM NEEDLE STARTING TO DROP WITHOUT ME ACCELERATING,THEN MY CHECK ENGINE LIGHT WENT ON,THEN IT ALL CHANGED. ONCE THAT NEEDLE WOULD DROP TO 0 MY CAR WOULD SHAKE REALLY BAD AND COMPLETELY SHUT OFF.AFTER 3 MECHANICS AND ALOT OF MONEY SPENT, THEY WERE ABLE TO FIX IT. NOW 3 MONTHS LATER,IM STILL HAVING THE PROBLEM LAST WEEK IT STARTED DOING THE SAME THING WITH THE DIFFRENCE THAT IT HAS NOT SHUT OFF.