Car jerks when going past 2500 RPM

1992 nissan 240sx SE manual 83,000. The car can't idle unless you give sufficient amount of gas to keep it at a thousand rpm if you don't it sputters around 500 rpm and below, and when you start going if you hit 2500 rpm the car will start jerking and shaking until you shift. We changed the spark plugs but I think that one of the plugs aren't firing right, and there's oil leaking into one of the cylinders. What's up with it?