Overheating sometimes

[Fippe]

I have a maxum 2400scr from 97 with a mercruiser 5.7 freshwater cooling, using alpha one gen2 as water suppliant for the heatexchanger, I bought it in the beginning of this year,
After a few weeks i got tired of the clapping sound from the outdrive and drove the boat to a workshop,
they lifted the boat up and mounted off the outdrive, nothing was wrong except that some plastic shims were missing and that made the outdrive clapping against the gimbal.
When I picked the boat up and drove away it didn't take more then 5 min before it over heated.
Turned the engine off and tried to se if anything was wrong and I couldn't find anything,
Started her up again and continued my journey home, the trip were 1,5h long and i hade a speed at 7kn and no owed heating problem, for the moment I just thought it was air in the system after they had it off,
Then later this summer on my vacation it overheated a few more times, and every time it was just temporary, just like it peaked at overheating and went back to normal temp.
I found out that it leaked freshwater coolant and fixed the leakage and after that I blown my headgasket.
My question now is, how come that the engine was overheating in just a few seconds up to max 30 seconds?
If it was low on coolant shouldn't it overheat continuously and not just a few times,
Total this summer I used the boat for about 40h and almost never went over 7kn.
Could it just be a coincident that it overheated after it was at the workshop?
Now I have torn apart the engine and when I drained the engine on coolant there were no debris in the coolant, just a clear blue liquid as it should be.