Took the Merc 15 (2002 2-stroke) our after a few weeks of not using it and it was working fine, but about 3-4 minutes into my runs at WOT, the motor will jump from 5200 to 5800 RPM and slow down considerably in speed. I am currently using a SS 10 pitch prop. At 5200 RPM at WOT I am getting about 17-18kts, and at 5800RPM at WOT I am only getting 13-14kts top speed. The motor sounds audibly revving higher, but the speed drops down like that on my GPS. It was also the first trip out on the new hull, so the prop was blowing out on turns that I was testing purposely. After it 'blew out' it would rev high like that and stay there as well. I have sincle lowered the jack plate about 1" and I will be re-testing it today. I tried the outboard in all 3 tilt settings and nothing changed. Is it going into protection or something at that high of an RPM? it seems that 5800-6000rpm is the max for this motor.