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My name is ***** ***** I am a Expert here at Just Answer. I noticed that your question was not getting a response and thought I would see if you still need help with this. If the auto parts facility had pulled a code out of the vehicles engine computer for an oxygen sensor fault and with what you have described here this sounds like this may be a fault with one of the oxygen sensors and most likely the one before the catalytic converter itself.I have seen this sensor fail before and cause this type of fault as it will read incorrectly and can cause a lean or rich running issue and can cause what you have described. It may be a good idea to have this sensor changed out to see if it corrects this issue.This may be an intermittent sensor failure and this may be why the dealer had seen it as working properly. I would also have the fuel pressure tested as a weak fuel pump can also be common and cause what you have described and can also set an oxygen sensor code for a lean running condition. Thanks Pete