We have a myson apollo boiler with 2 ... 2 port sunvic valves

When it runs on CH alone it works fine.
When it runs on either CH and HW or HW alone the boiler fires up for about 15 minutes, working normally then starts shutting down. The cycle of firing up for 20 secs then shutting down for a minute or so then firing up again would continue if we didnt turn it off.

We have checked the sunvic valves and they appear to be working ok as does the pump. The flue isnt blocked.

can anyone throw any light on this problem?

Thanks in advance