Boilers with low water content are prone to flash the water in the heat exchanger to steam if the water is not flowing fast enough to carry the heat away at a temperature below boiling. If the boiler flashes the water to steam, pumping stops, so the radiators get cold.
Investigate the pumping rate through the boiler to make it fast enough to carry the heat away at higher settings. This would be indicated by taking the water temperature entering the boiler and leaving the boiler. The difference in temperature should be less than 30F or 17C degrees.