A Couple Of Questions / Suggestions :
Is The “Check Engine” or “Service Engine Soon” or other instrument cluster warning / information lights illuminated now or have they been ?
Has anybody including the dealer ever mentioned checking for or finding any trouble or fault “codes” in the process of diagnosing the problem ? If so, do you know what they are or are they written anywhere on a receipt or anything ?
Have you taken this car to an autoparts store like an Advance Auto Parts or Auto Zone and asked to have it checked for any stored OBD2 codes ? Many do this for free out in the parking lot. Some folks post the codes on this site as clues as to what the problem(s) might be and people offer suggestions.
Should you get some codes, write them down and post them here in a format that looks like: PO100 or PO304, or PO136, etcetera. Maybe somebody can offer more specific ideas.
CSA