Sounds like the fuel pump is having an issue keeping up w/ the demand for fuel. That would explain why you decelerate when you push the gas because all that air is going into the cylinders and not enough fuel. Check your Fuel Pressure Regulator by checking to see what kind of fuel pressure it is holding. Also, there could be an issue with the pump. I don't know much about the 420a motor so maybe Jason or Dave can chime in with some useful info for ya.