2002 Buick century,

Most vehicles, there is not much load on the engine in park or neutral. There is a load when you put it in gear. How long since a tune-up? Any applicable trouble codes? Check and clean anything that has to do with idle air system. Clean the throttle bore and throttle plate, clean the iac and small passages in throttle body, see if it helps. Check the maf sensor where applicable. Don't replace any parts unless your testing points in that direction. good luck

replace gas tank sensor

REPLACE GAS TANK PRESSURE SENSOR

any codes ? Those problems should throw a code .Look for loose/broken vacuum lines .