It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. Is there a Check Engine Light/Service Engine Soon light on? If so a scan tool will be needed to retrieve codes stored in the computer of your car. The codes stored in the computer would indicate where the problem is or at least give a basic starting point to do tests. If so the computer in your car is going to have relevant information stored in its memory which will help figure out what is going on. A scan tool is needed to retrieve this information. A professional grade scan tool allows key engine sensor data to be analyzed right off the scan tool without having to do intrusive testing at each sensor.I would be looking at fuel pressure, fuel trim figures on the scan tool, look at what the computer sees as being the air intake volume into the engine to see if the throttle body/air flow meter is supplying correct information to the computer in your car. There is a professional service called Identafix. http://www.identifix.com/info/about_us.htm They only deal with professional repair shops. They are in the Minnesota 651-633-8007 begin_of_the_skype_highlighting 651-633-8007 end_of_the_skype_highlighting. Your auto repair shop do all their diagnosis tests, and discuss your problem with a team of professional technicians great service cheap and very bright people. If the repair shop that are repairing your car are stumped this is a great service. This company only deal with automotive repair shops and don't answer questions for the general public but are an amazing knowledge base.