Ferroli Modena 80 problems?

When I switch on my Central Heating, with the pressure at 2 bar, the system continually switches on & off for at least 1 hour.
The pressure slowly rises to 3 bar & then the heating works well, but water is expelled from the "overflow" pipe.
The hot water system works sometimes, but not always.
The next time the heating is used the pressure is below 1.
However, at no time do any warning lights come on.
The unit is 7 years old & has not had any new parts fitted as yet.
Can anyone help please.

Thankyou
John