Car started shaking on the highway but only during idle

I pulled over to the side and when I tried to turn the car back on it would only crank but would no fully start. I towed the car to the nearest repair shop and they told me it was just carbon buildup and that since they did not have the machine to fully clean it out, I needed to take it to the dealership. I took it to the dealer and they told me it was the injectors. I trusted them and they replaced all six and did a seafoam treatment on the engine as well. However once I went to pick up the car, during acceleration there is a popping noise. I told the mechanic about this and he told me that it was just a bad converter and that it might need to be replaced. My issue is, I had the catalytic converter replaced only a few months ago and the noise was never there until the dealership 'fixed' my car. So what should I do at this point?