2003 Yamaha 115hp 4 stroke outboard

by Glenn B
(Montross Va USA)

Once the engine heats up, like running it for 15 minutes at 25mph, it starts to sputter and hesitate, and sometimes even stalls at running speed. The tach goes crazy up and down when this happens. Once it stalls, it might start back up, but usually won't start until after 30 minutes or so. It runs poorly from then on until it cools down.When it's cool, it runs beautifully. I thought I had water in the fuel and dumped out my boats 10 micron filter into a glass jar, and no water. I replaced the filter anyway. The float on the motors filter shows no water. My #1 spark plug was black, so I changed it and the injector and all checked great until about 15 minutes into my cruise ck, and the motor di it's hesitation, sputtering, stall. Tach all over the place. No overheat alarms, and I just changed the waterpump and thermostat at the beginning of the season.
I'll take any suggestions please?