Answering some questions may reveal the cause:
(1) is the pressure always below 2.5 psig on a gauge that goes to zero when the burner has not been running for more than 15 minutes?
(2)Is the radiator valve open completely?
(3)Does the boiler have a Hartford Loop?
(4)Has the boiler low water cutoff been disassembled, scraped clean, reassembled and tested as part of the required annual service?
(5)Is this a new problem since a new boiler was installed, or has it always been this way?