Ok so I bought this 2007 Chevy cobalt Lt 2.2L almost a year ago. The lady I bought it from sister got the converter replaced at a muffler shop about 3 years ago before she passed away. Then it sat in the garage for 2 years. Anyways about a month and a half ago it put out a p0420 code but had absolutely no problems. The next day it hesitated when I hit a bump,like a jerking motion. The rpms went down to like 500 and then right back to 2500 in less than a second. Now it only does that when it's above 75 degrees and it's random. It doesn't happen everytime I hit a bump. I've replaced the spark plugs,cleaned the fuel system out,and cleaned the mass air flow sensor which I'm going to replace this weekend. I was wondering if it might be the coil packs?