If its a bacteria bloom is it possible that you are overfeeding when it occurs. Does this happen after you clean your filters ? It's weird, usually you shouldn't have any bacteria blooms in a cycled tank unless something happened to your bio filter. Maybe you had some power outages ??
-- Edit -- Next time it happens test your water before any water changes for ammonia, nitrite, nitrate, and ph. Water tests tell alot in these situations.