Myson Apollo 40S keeps shutting out!?

when I switch on the boiler (it's usually switched off when hot water is not required as the fan is noisy and goes on my nerve!) from the main, it works for a while just fine and when water temp reaches to some level (this differs from time to time and my guess is that it depends on the temp of water because the timing each time is different) it shuts itself down and fires up again within a 1-4 seconds. From this point it works for a very short time say about a minute or two and then shuts off and so on. I have replaced the thermostats (High, Low and limit) but nothing changed!
Pump seems to be working just fine and I've already clean the system with Fernox and drained the system so I wouldn't think it is circulation. (I might be wrong though!)
Any comment will be appreciated