If pressure is dropping, you have a leak. Check all fittings out of boiler. Check around the fittings on all radiators, common one is leak from the top of a thermostatic valve, just under the control dial. Also check the top fittings on radiators, the bleed screw one edge the blank on the other. Topping up that often it should be noticable, If all radiator fittings are ok, next to look for is damaged pipe work, or soldered fittings. But you have a leak.
SD, January 2012