2008 Chevrolet Cobalt Check Engine Light

In July, my check engine light came on. The car started to hiccup as I was driving it. I took it to my mechanic and it said it was the crankshaft sensor. I replaced it and the car worked fine for a about a month. The check engine light came back on. The computer said it was the crankshaft sensor again. Thinking the first part, was a after market part, I went to the dealership to get the part. Same thing happened. It worked okay for a short while then the check engine light came on. The car was getting difficult to start and when I took it to my mechanic, he said the computer is still saying crankshaft sensor. He replaced the cam sensor at no charge, because he was convinced it wasn't the crankshaft. It improved slightly, but I know it is not right and it is still not starting well. Once again the check engine light is on for the crankshaft. I should also mention when the problem started, the tachometer worked on and off. I hope that is detailed enough. Thank you for your help on this.