Galvanized iron pipe has a relatively short service life in potable water supply lines compared to any of the other commonly available piping. The galvanized coating is never perfect, is cut away or damaged at the threads, and potable water has some dissolved oxygen in it, constantly available to rust the exposed iron. IMHO, I'd never use it on potable water, although it can be used in certain HVAC applications. Your house may have been built when there was a world-wide copper shortage, and the prices spiked, making builders look for a less expensive option (I don't remember the timeframe, but it happens periodically). The only way to fix this is to repipe the house...it will only get worse. The worse point will come when the internal has rusted enough to reduce the volume, the flakes end up clogging the aerators and damaging the cartridges and washers of valves, or, just starts to leak though pinholes. The next earthquake might just shake things enough to give you massive problems.