Ferroli Modena 102 Boiler - oscillating water pressure?

I have a Ferroli Modena 102 Boiler. When I run the hot water at full bore, after a second or so, the pressure drops from 1.5 bar to around 1.0 bar and the needle on the pressure meter starts oscillating violently. The water temperature also drops. At the same time the boiler makes clanging noises and pipes start to bang. I do not appear to have this problem in the summer when the heating is off. The pressure returns to normal a few minutes after I turn the hot water tap off.

Could anyone assist?

Thanks
David