I fixed this on the weekend. The gate valve on the exit of the hot water tank coil was too restrictive, slowing the flow down too much. The boiler didn't get rid of it's heat fast enough, and so it shut off, for a few minutes.... Then it came back on, off, on, off etc. It never locked out, it just didn't do a big long burn.... Backed off that valve a turn and it is fine.
