I'd keep flushing it, and temporarily remove the heater control valve to get the heater core well, too. Courtney from the now defunct Cortney's radiator shop (A Good Place to Take a Leak) here in town told me a long time ago that the proper way to flush the system is to get it up to operating temp so the t-stat is open and undo the lower radiator hose into a drain pan. I was always too much of a pu$$y to try it, though.