Get a thermometer on the flow away from the heat exchanger to see what the water temp is getting up to.
If its getting up past the 80oC then could be the boiler stat/ thermistor letting it run to long hence tripping the overheat at 100oC
Also could be a slugish pump not getting the water away from the boiler fast enough or some sludge in the boiler.
If its getting up past the 80oC then could be the boiler stat/ thermistor letting it run to long hence tripping the overheat at 100oC
Also could be a slugish pump not getting the water away from the boiler fast enough or some sludge in the boiler.