This is a low-mass copper fin boiler with cast iron headers and will act much like the newest low-mass high efficiency condensing boiler if minimum flow rates are not maintained while the unit is firing; it will kettle. Most boilers will require minimum flow determined by the pressure drop through the boiler regardless of calls for heat or circulators running. If it goes to high-fire on one long or restrictive zone with single circulator it may kettle. Inversely, if all zones call and all circulators are running energy is wasted and velocities may cause other problems.
If it worked fine before we would look to the circulator performance. We prefer a single pump with differential by-pass or ECM pump properly sized and never push an expansion tank, scoop and vent as you see here and so many other places.
Most two-stage gas-fired boilers step-fire on temperature.
By the looks of the boiler it is time to shop for a new one.
PS. Air in the pipe will result in poor heat transfer and often makes low-mass copper boilers kettle.