Is the car in good shape physically??? Sounds to me like you may have a clogged fuel filter, often over-looked. Is your car in proper tune?? When fuel doesn't burn thoroughly enough for your catalytic converter to do it's job, the converter may clog up. There are special chemicals available from many auto parts stores that you can pour down your throttle body with the engine running and it should blow the muck out of the converter, if it is clogged. Never use cheap non-tier one fuels with inadequate detergent ratings, as they will clog the thing up all over again. Try the fuels that advertise "drive your engine clean".