I would have made changes in my design before I replaced the unit.
First, the problem is not that the unit is too small the problem is the distribution system probably or thought out. was not well designed
Second, Zoning is critical having more zones, where not all are calling for heat at the same time will save you 20% of the capacity.
Second the 520 does not have enough storage of hot water when the pumps are turned on. The first return of cold water to the unit is probably too much for the unit . The temperature drops and the triple aquastat says it to too cold and shut down the pump until the water temp is up.
Solution add a secondary in line insulated water storage to be used in the winter which is heated by the Boiler at 180 degrees (Not just preheated) this way you have twice the volume of water to circulate. As the season get warmer it can be bypassed if valved properly.A much cheaper solution than replacing a boiler.