My 1977 Mercruiser 454 started running poorly at higher rpm, not going above 3000 rpm, stumbling with the occasional backfire. I tee'd a vacuum gauge into the fuel line before the pump and read 0.5 in Hg at 1800 rpm and between 0.5 to 1.0 at 3000 rpm with the gauge needle vibrating fast. I'm guessing I need a new fuel pump?