vokera linea 28 - intermittent hot water?

when running the hot taps - after a minute or so the boiler shuts down (ie no flames). If left the water runs cold then the boiler kicks in again and hot water returns for a minute or so, this repeats. It's been suggested to me that the problem is a diverter valve. The few plumbers/engineers I've asked don't want the work, seems they only fit new systems these days! They don't repair. I've found a manual on line but it seems to look totally different from my (possibly older) model. Any help available out there? System is filled to just over 1bar and water flow is 8 litres per minute.