[hermanpj]

check your fuel filter as well. your car's 7 years old may have some crap fuel sediments in tank. did you ever maybe let it run down a little too close for comfort, or maybe even did it run out of gas? this can ingest crap into the fuel system, which the filter should catch. however filter can get clogged.

using wrong grade of gas can cause pinging as wrong octane level affects fuel ignition timing. engine managment system can handle timing adjustment to an extent so it doesnt always cause problem. i'm not an expert on turbos, but if your car has a turbo, i do understand that using a lower grade gasoline than recommended (with less octane) will result in harm to the turbocharger as exhaust gasses do not match what the engineers have designed into the turbo. that being said, what you are describing sounds to me more like either a problem with fuel delivery (injectors or filter) or spark.

what happens when you bury your foot in the pedal? does it just do nothing/sputter/jerk? do you ever get a 'check engine' light on the dash?