How to sort this boiler problem out (majorca 240/6 back boiler/frontfire)?

How to sort this boiler problem out (majorca 240/6 back boiler/frontfire)?
I have a majorca 240/6 back boiler/frontfire. About 5 years ago I had to have the central heating on all the time in order to get any hot water. I found that one of the two port valves burnt out (open circuit) and the control timer was burnt on the circuit board, so i renewed both two port valves and the control timer. I also changed the pump and thermo coupling. The heating and hot water system then worked fine.
The same thing has now happened again, so I have renewed both two port valves and the control timer. I did not renew the pump as it is working fine, but I did renew the thermo coupling. Now when you switch on the boiler, it fires up, runs for about 3 to 5 mins then cuts out. The pilot light is staying on all the time. Any ideas?

Many thanks, Baggy


Check the header tank has water in it.Could be a blocked cold feed. Tony

thanks Tony
checked water supply header tank all ok it does seem as if there is an air lock some where, but i cant see where? rads all vented, hot water vents through all the taps?
any more ideas?