Or it may be as simple as it was for me on my T6-354 the thermostat was not sealing arond it self in the housing. Opened up and installed a small o-ring to seal properly and after that engine keeps correct temp regardles of load!
Summers in Scandinavia only last about 3 months and boating will always be priority one, but now and then I take my ''second hobby'' for a spin :dance: