98 Contour Shuddering/Stalling/CEL

All of the testing came back to it saying the mixture was to lean, upon checking the oxygen sensors, the one after the catalytic converter (exhaust?) was completely covered in black soot, i cleaned it out, reset the computer and it ran fine for 2 days before the CEL came on again. When coming from high RPM's in first or second gear and then gradually breaking it would stall out, which isnt fun while turning. Anyhow, the ODB or OBD (right letters, wrong order?) tester showed once again that the mixture was to lean, and i checked the oxygen sensor again, and it was covered in schmootz. So i went to autozone and bought a brand new Bosch Oxygen sensor, and put it in. Reset the computer, and it ran fine for about a day or too. Then it stalled out on me again.

My car has 77433 Miles on it, and i just gave it an oil change, and checked all the levels on the liquids inside.

Accelerating it works fine, but when I let off the gas to smooth out my speed, it shutters like hell. And when I stop and get out it smells like dirty socks.

Basically the question is this. What should I do? Would a Tune Up Solve this? It seems other people have had this problem, and discuss a vacuum pump, how can I check that as well?

Thanks For the Help Gang!

I would give the MAF a good cleaning and see if that helps. Check out how to do it here: http://www.contour.org/mods/mods.php?s=howto&displayid=36

Also, you may want to pull your plugs and see if theyre fouled.