Yanmar 6LPA engine rpm kangarooing under load

[neil reid]

Gents,
Needing guidance. When out cruising my boat suddenly stopped. After getting it started again i found that upon giving it RPM's under way it would go to approx 2200 rpm and then drop off to approx 1200rpm for a few seconds before going up to approx 2200rpm. This continued all the way in until i got into the marina and moored up. ( limped home)
In idle there is no issues revving the engine and it is as sweet as a nut.
When i go into gear and apply power i get this jumping up and down with not being able to get above the rpm mentioned above. Max speed about 8knts when normally 30knots.

I have changed fuel filter, air filter, oil filter, fuel/water filter with still the same effect. Prop spins freely by hand and no build up of growth as it has only gone into the water, Fresh water lake. Never been in salt water. There is no smoke in either idle or when i try to put in under load. The engine is a 2004 Yanmar 6LPA-STZP. Never had any issues before with power. This happened all of a sudden. Any ideas as to the issue before i pay a fortune to get it figured out. Thanks all in advance. Neil