Ariston Microgenus 27 MFFI?

Help! Noone can fix my boiler. it has a mind of it's own. It comes on when it likes and switches off when it likes.

Turning on the hot tap sometimes switches the boiler on to heat the water and sometimes it doesn't. And when it is heating the water it just cuts out like you have switched the tap off. Likewise with the heating. If it is in the ON position (i.e. not timer mode), it comes on sometimes and sometimes it doesn't. And sometimes when it is running and hasn;t reached the temperature is should it cuts off like you have switched it off. It may come on unexpectedly and run for a few minutes, sometimes longer then cuts off again. It even comes on and heats the radiators when the Timer switch is in the OFF position.

My various heating engineers are bamboozled and it has cost me a fortune so far replacing parts that they THINK it is, and it simply isn't.