Boiler keeps switching off?

I have an APPHA CB24X COMBI BOILER. My boiler starts working ok, but after a while it switches itself off. and a yellow light flashes every few seconds. Then it starts working ok, then goes off again !! The manual says the yellow light indicates a temperature sensor fault. I had this problem when under warranty and it took the engineers 3 visits to sort the problem out. Eventually they changed the flow switch and thet solved the problem. I have changed the flow switch myself and it has not made much difference. I spoke to a plumber and has said it may be the thermistor or overheat sensor or termostat. I seems the boiler works ok then cuts out. This cut out is the same for hot water or heat. Also I have recently found out that the pressure seems to drop on boiler and I have to keep topping it up, but i cannot see any signs of a water leak - Any Idea's would help please !!