94 Skylark troubles

I have a 94 Skylark with 89000miles and I have a problem with the idle at start up. After starting it I have to rev the engine for 10 minutes before I can get the idle to stay steady. If I try and put it in drive before I get the idle steady, the car will either shutdown or it will sputter until it gets warm enough. Lately I have notices that the smell from the exhaust is horrible and when I turn on the A/C and I can smell it too. I have taken it to a dealer and they can't find anything wrong. (Which cost $125 just to look at it). I had the radiator and transmittion flushed. I had a new catillytic converter and muffler system put on. Had a tune up. I am up over $500 and still it is acting the same. I need help.

sounds like o2 sensor or throttle position sensor