My 1990 prelude SI (B21A1) just started doing this yesterday. It still idles at 750 rpm, but shakes badly. Ive noticed if I rev up the engine a bit or start moving the shaking lessens. In addition, the exhaust sounds different, sort of a puttering sound. The car also seems like it has less power, which leads me to believe that one of the cylinders isn't firing. Any ideas as to what would cause this to happen?