sounds like flat or faulty expansion vessel. they have rubber diaphragm seperating air section from water. if that holes etc water will replace air filling the tank and therefore leaving less room for expansion when heating on.
eg cold pressure 1.5 bar----hot 3 bar + then leak from p r pipe and settle when cold to 1.5 bar or less. this is why you have pressure loss too.
thats my opinion without witnessing. if the boiler is located out the way somewhere not seen. fitting remote vessel rather than replacing boiler one generally cheaper repair
iain
March 2008