Bryan,
I keep my rodent shed at about 70 degrees, if not cooler. I got all my original mice from Ryan Hoyer, and he has had the same problem with them for years, it seems to be poor genetics. I'm waiting until Ryan has a good number of S-W's and I'm getting them from him, it's a lot cheaper that way . Are you working with a specific line, of just pet shop mice?

-Evan