jetta spark problems

this could be a connection or coincidence

it all started when i put some marvel gas treatment in my gas tank
the next day the car was driving great then sputtered and stalled on me.
i took it to my mechanic and he replaced the ignition coil,the car started ok but sounded rough in idling -we thought maybe the engine was flooded from me trying to start the car so many times after it stalled

anyway i took it home and ever since it has a problem starting. Sometimes it starts and sometimes it doesnt. I took it back to my mechanic -he re-installed the old coil and the car wouldnt start at all. so we believe it did need a new coil. but my mechanic also noticed that the first spark plug loses its spark after the car starts and the gas pedal is pressed.

any suggestions???

I'd just get new plugs and wires...

This sounds like my problem. the only thing is the shop has had my car for over a month and they say nothing is wrong? good luck.. All I know it runs rougher at idle then it dis with my old wires.