I have a Yanmar 4JH2-TE Engine and a Kanzaki KM4A Transmission. In neutral I can rev the engine to high RPM's when I put it into gear and throttle forward it barely goes up to 600 RPM's. I have to go into neutral and then forward several times and then when it grabs it goes...fast! This is an intermittent problem. Anyone have any ideas what the problem could be?