[Silvio]

hmmmm,
Sounds like you are checking all the right things. These diesels are pretty simple creatures when you get down to it. What tells you that you are over heating? Is it the gauge reading only or do you see and smell an over heated engine?

I don't mean to sound obtuse but are you certain that you are over heating and not that you are having a gauge problem?

Have you tried removing the thermostat and running the engine with it taken out of the picture (essentially in the always open position)?

Are you in particularly warm water that is interfering with the heat exchange, maybe causing a higher than typical reading but not actually over heating? Good oil level?
Good air flow around the engine cowling (not piled with gear) ?

Again, I'm sure you already checked these things but just throwing them out there in case.

Good luck.