21 Boat...I understand where you are coming from and I appreciate it. But the first comment made by DMD was "furnace is Buderus"....they only make hydronic equipment. Many homeowners don't know exactly what makes their heat work, let alone the source of it...With water and air and steam and electric....there is radiant, fan forced, hydronic and I'm sure more designations I've missed. At any rate DMD isn't the expert, you are. If you walk into a customers house and start correcting their "lingo" is that good business? At any rate....
DMD, all of 21boat's suggestions could be your problem.....It's clear he's versed in boiler symptons and repair. I suggest you get a hvac tech who knows boilers to help you. Boilers are great and can last 50 years (had a Weil McClain that did) but they just like all equipment need to be serviced and maintained.