Since everything else sounds fine, I would think it is the temperature, because it's rather on the low side. And you mention it can dip lower than that, which is definitely too low for success.

The bedding sounds fine, but I have no experience with it, so I can't say wether that could be a problem or not. I use pine shavings and moderate temps (70-75ish?) and haven't had any respiratory problems in years in even a single animal.