2002 Chevrolet Monte Carlo please help!

Ok so I bought this car about 4 yrs ago the transmission has been rebuilt from previous owner. Since then I have put all sorts of new sealant, valve gaskets for tranny and engine. Serviced tranny cleaned pans the whole works with full tune up wires and spark plugs etc. Fuel pumps filters, flushes to radiator list can go on and on but I can NOT for the. Life. Of me get my car to stop shaking while stopped at a light. When turning my steering wheel it sounds Like I'm need power steering fluid, or when I'm at a dead stop and try to take off it hesitates to go but it will catch up. If I drive in traffic and am stopped it sounds and feels as if the car wants to cut out but catches. It's self and stays on. My horse power isn't there. My check engine light comes on stays for couple says and turns off by it's self. I checked it once and it was for a catalytic converter or another I think. When I bought the car I kind remember the lady saying that 1 of the O2 sensor was bad I plan on replacing both and cleaning my MAF SENSOR to see if that helps what eles can I do before giving in and spending 1200$ on a new exhaust whatever it's called