Sime format pressure goes up and down?

Hi all,
My pressure guage shows up to 3.5 bar and stays that way for a long time and then just goes down sometimes to zero and then up again.
the boiler is working ok, I hear it firing up and the water is hot but not hot enough.
there is also a constant leak from the bottom coming from the small water exahust (with the long clear tube). This water is brown with corosion.
There was a technitian here 2 months ago that replaced the expantion container (??? sorry, free tranlation from hebrew) but that didn't help at all. Can anyone help ?
10X