I have a 1989 Mercury Mariner 50 hp motor. The motor starts on first turn and runs strong except that it sputters/shakes when i accelerate slowly. If i accelerate or decelerate while its shaking it stops. Also the top end speed has slowed, i used to hit about 35 going full throttle and now i can only get 30. I can tell by the sound that its not quite hitting its highest gear. Ive replaced the spark plugs, gas tank, and fuel pump so i dont think its fuel related. Any help would be appreciated. Thanks