Pawildman

Well Harold, case separation can be a number of things, as I'm sure you know. Your posts indicate you are a knowledgeable reloader, and I assume you are up on most of the possibilities. But just for the sake of rehash, let's look at some things. You said the cases in suspect were once-fired. Are you sure of that? Sometimes "once-fired" brass is sold at close to the end of it's acceptable life. Going on, I guess it's possible that the brass was fired in a minimum chamber gun, and you may have a max chamber. I certainly imagine you resized the brass before you started to load. Did you neck size or FL? If you had your dies set for your gun, did you check to see if the shoulder of the cases was being met by your dies? If not, ex. headspace may be present, promoting case stretch, hence early separation, particularly if the brass was indeed much more than once-fired.
AS you stated, I'd certainly try the virgin brass and see if the problem still exists. Knowing you and your posts, I don't think it will, provided you set the dies for that particular gun, if that's what you do. one other possibility is that the brass may just be old and brittle, but I've yet to have that happen to me...