In addition to the hammer spring, you might want to check the firing pin. There is a limiter on the pin to prevent too much protrusion. There could be a slight incompatibility between the bolt and the new pin. Your headspace could also be just a tad large, or the previous owner could have dry fired the gun enough to peen a dent in the chamber resulting in the case head not being properly supported at the point of firing pin impact. The broken firing pin was probably a result of dry firing.
