2001 s60 2.4 turbo. really hesitates at take off, does fine when feeding a lot of fuel to the engine but when i hit a constant speed, it starts to hesitate again, if i push on the pedal it goes away but once i hit a constant speed or slower speed for a short period of time it begin to hesitate as if it is not getting fuel unless i am constantly pushing on the gas pedal. i have no check engine lights, have replaced the fuel filter, spark plugs and coil packs. any suggestions?