overheating?

ferroli modina 80e
the pressue release valve was leaking all the time and and to refill it every day.
I renewed the valve head part no800130 but this has not worked properly.
I then notice that the pressure gauge was going from 1.2 bar to 3.5 bar.
I turned the heating knob to low but that worked for 2 days before I had to refill the system the hot water was only getting warm now and running slow.
please can you help me
thanks pete