thank you to everyone who tried to help! The issue has been solved
and I would like to share with you (and for future reference) what the cause was.
In few words, it was a over-heating problem due to the heat exchanger
being badly clogged with deposits and the fresh water
pump being unable to reach enough flow to allow for a proper heat-exchange with the flowing raw water
and therefore cooling
of the unit. Probably the previous owner had used inappropriate coolant
(like simple tap water) or something else happened who caused the heat-exchanger to clog on the fresh water side.
The problem wasn't easy to diagnose because we had a regular flow of raw water
and the unit run until load was applied, but it was in fact stopping as soon as put to work. We don't know if applying the load triggered the over-heating sensor or there is some other logic to it.
The heat exchanger
has been disassembled, cleaned in acid bath and re-assembled, which has made the Panda run properly under full load.
On Monday we will also install a new fresh water pump (apparently the extra pressure from having to work through a clogged heat-exchanger cracked the old one which is now leaking a bit).
Hope this helps. Thanks again everyone for your answers. Such a great community!
P6 6KW marine diesel