Having problems with Vokera 28 linea boiler - hot water off and on?

My problem is that my vokera 28 linea boiler is playing up. When you put on the hot tap the water comes out very hot to start with then cuts out and becomes stone cold before gaining the heat steadily until again it suddenly drops. The boiler itself seems to cut out at a certain temperature when the water is on. Although the water temperature at the boiler is set at about 40c the actual temp of water that initially comes out of the taps is much higher. We've had an engineer out to this, the boiler has been thoroughly serviced and the main heat exchanger cleaned but still the problem is persisting. Engineer thinks that maybe the main PCB or the thermostat is the problem but not 100% sure. Any advice would be greatly received.