you could have a leak somewhere hidden like under the floorboards, it doesn't take much water loss for a combi to drop on the pressure guage.
the other cause is if the boiler is over pressurising and dumping water out of the back of the boiler. find your boilers flue outside the house, below it somewhere should be a small length of copper pipe, check to see if any water is dripping from it or has been dripping.
call out a heating engineer, have the boiler serviced and ask him to investigate the problem at the same time.