Check engine light is on?

About a month and half ago, my engine light came on. I took it to a store and had them diagnose it by checking for the code, and an oxygen sensor code came up. When I took it to the mechanic to have it checked out, they said nothing was wrong with my oxygen sensor, and that nothing was wrong with my car at all. They turned off the engine light and sent me home. A week later, my car started idling high, between 1500-2000 RPM. It was very difficult to get the car to stop while braking, it was stuttering and lurching forward. I called a different mechanic to have it towed. They came to tow it, fiddled under the hood, said some plug was loose and it should be fine to drive now, and once again I was sent home. Four days later, the exact same stuttering problem came back. I took it to the third mechanic, they checked my car, couldn’t find a problem, but cleaned the throttle. Car ran fine for about a week, when the stuttering problem came back. Tried a fourth mechanic, who said the idle air control motor was sticking. They cleaned it, and the stuttering problem came back a few days later. I took it back to the same fourth mechanic, and they completely replaced the IAC motor this time. My car ran fine for a couple of weeks, then my engine light came back on. I took it to a mechanic, and they kept my car in overnight, looking for the problem. They seemed to have a lot of trouble diagnosing it based on the code that came up. The mechanics checked for a vacuum leak and were unable to find one. They claimed I needed to replace the plastic intake manifold. I was hesitant to do this considering what they said it would cost at this place, and my symptoms didn’t seem to match up. My car is still sometimes stuttering and idling high, I’ve smelled gas a few times when driving, and this morning while I was driving I was in idle and my car started to shake, which it has never done before.