My car's mixture was adjusted to spec,but "Mr. Oxygen Sensor" was so old that he basically was dead and caused my car to run poorly when the mixture was properly adjusted.
A number of factors/faulty components would cause the same symptoms.
Systematic testing will help you troubleshoot the problem and will stop you from throwing unnecessary parts at the problem.
The O2 sensor can actually be tested at it's connector under the passenger side carpet with a voltmeter to confirm if it's faulty.
Do a search, there have been many discussions with regards to the O2 sensor.