Aware of all the issues with these engines. Have just picked up boat from the repair shop for a very persistent problem of a midrange mis. Mechanic changed plugs to champion QC12PEP per TSB. But does not know if this will fix, guess he thought I had money burning hole in my pocket. I have changed fuel fillter and thermostats, use XD50 oil and 2+4 fuel conditioner. Diagnostics show OK and no water in fuel. Motor starts and runs good otherwise. Any ideas? Thank you.