I have a suggestion that is unrelated to your combustion problem directly but indirectly may help, bleed your uppermost radiators. Most hydronic systems the age of yours will have a central
Spirotherm vent or something like it, but air still somehow seems to get into the system and can become trapped in the radiators. This is especially true if the radiators are original vintage. Bleeding on old radiators is simple, look for a small vent valve near the top of the radiator and use a small screwdriver to open the vent long enough for the air to bleed out. You'll know when that is done becuase the vent will begin to pee out fluid instead of air. This simple bit of maintenance can make a huge difference in the efficiency!