When accelerating (kick it down) check engine light comes on and it misfires

I am having a misfire and a message on my stereo screen. The misfire was extremely bad when you kicked the car down as to pass someone. The check engine light would flash and the car felt like it would shut off right then and there. Took it to the shop. They told me that I need spark plugs. I had them changed. Not by them because I am basically a single mother and cannot afford $450.00. But I did have a licensed mechanic change them. Well the misfire is still there. He then hooked it up to a scan tool which showed a code of p0300. P0301 p303 and p305 I do believe. He then changed around the coil packs. Same codes on the same cylinders. So I took it back to the dealership told them what we discovered. An hour later they called said that it was two vacuum hoses and a ground to the engine was broken that was causing the misfire. So I went to pick it up. I told them I am taking it for a test run. I asked if they drove it. They said yes. I asked did they kick it down. Yes was the reply and all was good. No codes or check engine lights. Well I took it for a spin. Kicked it down three times along the highway. And on the fourth time. The check engine light came on flashing and the misfire. Misfire is not as bad. But the motor does not sound right. Almost like a fluttering sound. The check engine light did not stay on just flashes. But now I am getting the shutting down system to save battery on my stereo when I shut my car off. It also seems to start harder when you try to restart it. I did install a new battery in July. So it is only five months old. We did leave the car sit for ten hours because I know it has a battery monitoring system on it. And needed to be reset. My mechanic thinks it’s either the PCM or the crankshaft sensor. The only reason I take it back to the dealership I got it from is because it is under a wrap warranty and a lifetime power train warranty. I feel this is something that should be covered under one of those warranties.