while Ectoparasite infestation is the most common cause of this behavior, the flashing/scratching 'could' be something else. fish can be irritated by metals such as iron, copper in the water. If this is the cause, the fish seem to 'flash' most often right after a water change and less as they 'get used' to the irritant. Then the cycle is repeated at each water change. note that this reaction to metals is not the same for all fish all the time. have your water checked for heavy metals or ask your municipal water company for an analyses. This is a good place to start.
Fish can also flash in response to parasites other than the more obvious Ichthyophthirius (ich). Gill flukes for example which are not obvious externally but require a more internal investigation (gill scraping and viewing under a microscope). Gill flukes usually result in reddened, muccous covered gills.