We have a Worcester 350 Combi and is shutting down after 2-3 hours. ?

We have a Worcester 350 Combi and is shutting down after 2-3 hours with the pressure dial showing a drop to below 0.5. We have to add more water up to pressure and restart. This gets the CH and HW OK again for about 3 hours when the presssure then drops quickly, the boiler shuts down and the whole cycle needs to be repeated. There are no signs of a leak anywhere visible and the outside drain/overflow pipe is dry. Any ideas?