GTP 3.9L CODES P0050

2006 Pontiac G6 • 94,000 miles

My check engine light came on yesterday when I read it today I got 3 codesP0050 - H O2 S Bank 2 Sen 1 Heater CircuitP0300 - Random/Multiple Cylinder Misfire DetectedP0050 - H O2 S Bank 2 Sen 1 Heater CircuitAbout a month ago I took it to a mechanic for P0300 and had the rear cat replaced along with a radiator and a transmission pan seal. Since then I really haven't had any problem except the same one I came to him the first time. I have Tiptronic or the automatic trans with the manual shift option. (Dont really know what its called). If im in first gear and I push the gas the engine jerks like there is a transmission issue but my mechanic said there was no issue. Now I have these codes.
March 21, 2012.