The problem is *NOT* flow rate, it is the combination of the lack of adequate stored thermal capacity plus a recovery rate that cannot keep up with the rate of demand.
If it were insufficient flow rate, the temperature of the boiler wouldn't drop so quickly. The BTUs are being moved from the boiler to the house quickly enough, but they are not being replaced at the rate they are moved.
I must have missed where he posted what the boiler water temp actually is, at the boiler itself.
He needs more volume of water going the HX in order to keep the inlet and outlet temp pretty close to the same temp. If the HX is pulling too much heat out of the water before it leaves the boiler will not have a prayer of keeping up. Now if you add more volume of water to the HX you won't lose as much heat accross the HX so the boiler has a better chance at keeping up, while deliverying MORE BTU's to the plenum, at a more sustained rate.