vokera 20-80 rs pressure gauge drop?

I have a vokera 20-80 RS. Lately the pressure gauge needle drops violently when hot tap or central heating is turned on. it makes a thumping noise as hot tap or central heating is turned on. The needle would go down to 0.5 then settled at perhaps 1.0 (if the original setting is 1.5). I then I have to top up to 1.5. After this has happened the hot water or central heating would run normally. Sometimes after the thermostat temperature setting for central heating has been reached it makes a humming noise with slight vibration to the boiler unit. A month or two it has a leak as hot water is turned on but this has been solved by gas engineers. I called the gas engineer in to have a look at it and he said there is nothing wrong with the boiler and added that the boiler is old and parts would be difficult to get hold off especially the gas valve.