car is backfiring after complete tune-up

I HAVE A 1996 PLYMONTH VOYAGER WITH 121,000 MILES. IT AT FIRST WAS JUST STALLING ONCE IN A GREAT WHILE. MAYBE ONCE A WEEK OR SO. AT FIRST IT WAS AT A STOP SIGN, THEN WEEKS LATER WHILE DRIVING, IT QUIT AT A STOP LIGHT. I HAS A CAM POSITIONING SENSOR AND A CRANK SHAFT SENSOR ALONG WITH A COMPLETE TUNE-UP. IT SEEMS AFTER THE TUNE-UP, I STARTED HAVING THE BACKFIRING OCCUR. I HAD THEM RE-CHECK THE SPARK PLUGS AND WIRES AND WAS TOLD IT WASNT THAT. NOW IT WILL ALWAYS START BACK UP, BUT SEEMS LIKE WHEN IT WARMS UP, IT STARTS BACKFIRING AND JERKING. HELP ME PLEASE. I KEEP PUTTING TOO MUCH MONEY IN THIS.