My 2008 cobalt with 55,000 miles on it is suddenly having problems. Once in a while when I go to start the car, when I turn the key it is silent and nothing happens. Then I take the key out, wait a minute and try again. The engine shakes violently and my message center says "Engine Power Reduced". I take the key out, wait another few minutes and try again, it will drive fine but the check engine light stays on for a few hours. I brought it to the dealer today, I have the GM warranty plus an additional extended warranty on it, but the dealer said they couldn't find what was wrong with it and sent me on my way. Any ideas??