Why do my spark plugs keep going bad?

The spark plugs in my car keep going bad. About three weeks ago my car started to misfire when I went past 40 and then my check engine light came on. I took it to a shop and they said a gasket was leaking so they replaced a gasket, coil, and all of the spark plugs plus did an oil change. A week later I had trouble starting my car. By the time I got to the mechanic my check engine light was flashing. They said all 4 spark plugs had gone bad so they replaced them again and said they used fuel injector cleaner. I went back the night day because my car was idling hard. They told me to premium gas once to help with this problem, which I did the next day.

A week later I had trouble starting my car again. I went to a different mechanic. They said the previous company put the wrong spark plugs in and changed them. Two days later I had trouble starting my car again. The mechanic said he used fuel injection cleaner again and my car should be fine and he cleaned off the spark plugs because two looked "dusty". Here it is 4 days later and my car didn't want to start once again.

Does anyone have any idea what could causing this? I know it is not the battery or the starter because I had these checked.

FYI: When I say I have trouble starting my car I mean it takes about seven tries to the engine to turn over.