I have a Yamaha 115 4 stroke on a 2012 2275 gcw. Purchased in 2013, motor ran at 5600 rpm WOT since purchased. The last few weeks motor runs normal for about 5 minutes then rpms start jumping up to 6800 but motor sounds no different and speed does not change. These measurements are on the factory installed tach. Anyone have any ideas what is causing this?