My 86 Century T Type 3.8 SFI with 120,000 miles engine gets quite rough and lacks power under moderate or full throttle acceleration. The transmission shifts normaly, there is no stalling. With light acceleration and steady speed it is smooth as it should be. The check engine light is not on and I checked for codes and there were none. I replaced the spark plugs, wires and air filter. The catalytic converter is 2 years old. EGR valve is functioning. I listened to the injectors with a stethascope and they're all clicking as they should. Any suggestions what I should check next? ( a desription of any test procedures would be appreciated as well).