2003 Nissan Altima

I have an issue with my 2003 Nissan Altima. I have taken it to several mechanics with no luck and don't know where to turn. A few years ago I had a neighbor mechanic check out my car. He told me my air filter needed to be cleaned. After the cleaning my car acted crazy and would sound like someone was repeatedly stepping on my gas pedal. Everytime I would put my car in park my car would act like I was stepping on the gas pedal every 2 or 3 seconds. As time went by this issue went away. Every 4 or 5 months after than my car would turn off on its own. And A few months ago my car would shut itself off. At First I was told it was an issue with my battery again. I had purchased a new one. I ended up having the same issue. Sometimes, after putting gas my car would not start. I took it to different mechanics to get different opinions. They told me that it was my catalytic converter. After that was replaced I still had issues. Also replaced were the V-tech, Cam sensor. The computer system was reprogrammed then replaced when issues still occurred. The strange thing is when the cam sensor is taken out the car starts just fine. The mechanic said this was a sign of a short but he did not find it. The Nissan dealer wants to charge $150 an hour just to take a look. What do you recommend I do? Thank you for any help or suggestions that you have.