dies @ stoplight, idles rough

Please help. I repo'd my contour after my ex let it overheat and blew the head gasket. She had it "repaired" by someone. I would die, knock, and not accelerate after 5 minutes. Pedal to the floor and engine would rev but go practically nowhere. I took it to mechanic who said I needed a valve job. Got that done. Still a piece of crap. Dies at stoplights(worse with A/C on.) He replaced MAF sensor. Still knocks and dies. Fuel filter new, plenty of pressure, check engine light on and he says code says running lean, O2 sensor bad..replaced 02 sensor...still the same. He has given up and it hasn't changed.

sounds like it could be the timing

i bet the headgasket never even got replaced if it still runs like junk.

I need to elaborate. The knocking only occurs slightly on acceleration and worse when it enters overdrive. Also, since the last time it was in the shop I now have a distinct exhaust odor in the cabin

sounds like a bad valve. being out of time can make the piston hit the valve.or could be a valve stuck open. the timing belt could be loose. but not to where it will jump time.