I have two-week old chicks that are having the same problem and I can't figure it out. I will go down and check on them and everybody seems fine. Then an hour later I go down and one will be down. I give it water and food and in an hour or so it's up again and seems fine. Ultimately, however, all that have been down have ended up dying within 24 hours. I have had this happen with younger chicks, but never any that were over 2 weeks old. I have tried everything, including vitamins, a little Sevin dust thinking maybe they had mites, Sulmet in case of cocci, checked and rechecked temperature, made sure they aren't in a draft, etc. I just don't understand it. I changed out their entire cage yesterday and put in fresh bedding, started mixing in a little bit of a higher quality food, sanitized the waterer...just about everything I can think of. When I went down this morning, there was another one that didn't make it. The odd part is that some of the chicks that were hatched on the same day are twice the size of the ones that are having trouble. Could it possibly be something genetic? Just maybe weak chicks? They are light sussex and blue-laced red wyandotte crosses, which I have hatched successfully for about a year and they are normally VERY hearty, grow quickly, and thrive. This is my fourth hatch of this particular cross and I've lost over half of them. I am perplexed...and sad.