Yanmar 2GM problem

[sharkwatcher]

I have a problem with the Yanmar 2GM in my Hunter 31. First a little background info: The engine has 1200 hrs, has just had an oil change, all filters (air, fuel) changed and Trans oil changed. The engine starts great and idles smoothly and does not smoke at all and accelerates smoothly. Engine temp is a steady 180 F.

At around 1180 hrs I started noticing the full throttle RPM's decreasing from 3200 now down to 2500. I have not changed the wheel and the bottom is cleaned monthly. I have verified the rpms with an optical Tach, and checked the idle speed (900 rpms) and inspected the throttle linkage and adjustment and can find nothing out of adjustment. Any ideas on where to look next? Any help and advice is greatly appreciated.