I had exactly the same problem, in the end I gave up and took out a British Gas Homeserve policy and they came out straight away and fixed it all, its not cheap but at least you can always have peace of mind.
it is the water pressure, check the needle on the little clock and you will find it has dropped below 1, if so there should be a tap underneath which is the water inlet, turn this gently and leave open until the needle reaches about 1.5 or just above then shut the tap off(IMPORTANT).