When it happens, i opend the bonnet, checked coolant level, it was all right, and the engine was not overheated.

I think it may be a built in safety device to stop you causing further damage to your Merc.Check with a main agent as soon as possible.
Masha.

replace the in cap censor in the header tank the bottle where u put the water it has wires to it they get faulty with the heat/damp comm fail