Jaguar 28kw combination boiler?

hi i have a Jaguar 28kw boiler installed in my house and i keep getting problems with it. it is keep over heating and then cuts off. it starts back up again within a few seconds then cuts off again. the Central heating stays on all the time and cannot not turn it off unless switched off from the mains. that also cuts off and relights after a few seconds. i have changed the main heat exchanger as many gas plumbers have said that is the problem but i am still getting the same problem.
please could someone help me as i have tried everything and cannot fix the problem.

thank you