Yanmar 3TNV82 Troubles

I recently acquired a Tak TB 125 mini ex with a Yanmar engine. This unit has some real problems. I could use some help where to start diagnosing the problem[s]. The engine starts and runs fine at idle, at about half throttle it starts to puke out and actually drops in RPM. I have put in fresh fuel, cleaned out the water separator (full of sediment), replaced the fuel filter, changed oil and filter (oil was overfull). Additionally, the coolant looked contaminated with exhaust (blackish film in the reservoir) and there is bubbling in the reservoir as if there is exhaust pressure in the coolant system. So now I'm think bad head gasket or worse. Where should I start? Leak down test? This is my first diesel engine so no experience with them but i'd like to learn. I am mechanically inclined and if I could find a good manual should be able to do some troubleshooting and medium duty repairs Any help would be appreciated. Thanks

Yanmar 3TNV82 Troubles