The first thing to do is to have the engine management system scanned. There is probably a defective sensor that is confusing the computer so it is reducing the power. Any check engine lights? Does it feel like your exhaust is possibly plugged up from an over heated/defective catalytic converter?
Yes, in fact it is the computer that controls all the power, after it receives input from the driver, but if the input is skewed,( bad senors etc) then the computer will not control the engine properly, it is the old 'garbage in, garbage out' scenario. City driving alone would not hurt a Catalytic Converter. Do you have a Check Engine or Service Engine Soon light on? If so, the first thing to do is to get the codes scanned and addressed ASAP.