I have a 99 90 hp johnson. Ran great until two weeks ago. It has developed a miss. I changed plugs and fuel filter. It will run when first in water and then misses bad after a few minutes of running. It completly quit last time. I trolled back to dock.(1 hour). Ran fine when loading it up. I know it s time to see the mechanic. Just wondering what the damage may be? Thanks!