my wife was driving and the car started sputtering and stalled, she restarted and could not go more than 5 miles an hour, then pulled over and shut car off and restarted and it began running ok, except for downshifting erratically. when she got home i took it for a drive and it was totally fine but the check engine light is on, so i scanned codes and this is what i got. C1278, P2101, P1779, P1780, P1795, P0121, P0221. In that order can anyone tell me where to start, i replaced throttle body with a used one at beginning of summer, i cleaned it before installation. I remember having to replace it because of an erratic behavior, but it was different codes back then. Oh almost forgot this is a 2003 cts with a 3.2l

 

I'd first start by looking at the idle control module, The MAF maybe on it's way out. Have you cleaned the throttle body? Look around near the bottom of the throttle body for some vacuum lines, make sure they are connected and there aren't any cracks in them. I've also heard of where the Torque converter in the trans can make the car act like this but it's rare. Do the easy stuff first.