First lets figure out the heat load for the building. I suspect your boiler is over-sized, as are most. Given the size of the boiler we will endeavor to size a circulator to drive off the heat produced at the "good old American" 20° Delta T. In residential boilers with P/S piping this is usually a small pump on the order of a 007. Expansion is a matter of temperature and volume. It is very rare that a #30 will not cover the 3% expansion volume likely in a 180°F designed system. Most radiant floors use a lower design temperature.
Here, we are not so interested in the supply as the return (you correctly turned the boiler up to protect it from the fatal results of cold return water temperatures and sustained flue condensate) but it sounds like the high limit is turned up too high or is not functioning properly. If we knew what the design temperature should be (the temperature the water must be in order to satisfy the heat load on the coldest day of the year) we would naturally turn the high limit to that setting, say; 160°F. To let the boiler operate at a higher temperature would only serve to burn more fuel than necessary.
Unfortunately the Onix system was promoted as a staple up and insulate sub-floor system and often specified with a high design temperature. The first couple of sub-floor radiant systems I installed were staple-up PB systems--still working--but not ideal.
Look to the zones, as the boiler is clearly "boiling" and unable to get rid of the heat. If there is no load the water in the boiler will change phase regardless of the circulator since it sounds as if the circulator is circulating around the boiler and back.
Turn the high limit down (poor way to control a boiler) and make sure the zone circulators are taking heat away from the primary loop.
More thinking, less parts changing.