I think this car is too young to have plugged catalytic converters. My guess is on the oxygen sensors; I have heard that they can cause this kind of problem because the fuel mixture gets screwed-up. I am not sure whether a diagnostic tool will be able to tell you anything, but if you have one, hook it to the car and see if it returns a code. Although, I recall hearing, that an oxygen sensor failure will trigger a check engine light and you do not mention the display of any messages or lights. Also, as someone else said before, check the condition of your air filter.