Hi everyone. This one has me, and the mechanic, scratching our heads. I have a 2000 268 Islander with a 2000 Yamaha 250 2 stroke. When I run the boat trimmed out at say 5000 RPM's for about 10-13 miles the rpm's will all of a sudden start to fluctuate from 5000 down to 4700, back to 5000 and...