Pardee,
I have had a mom have a similar experience with milk spoilage - she used the
Playtex nurser bags.  I have not had anyone complain of spoilage with the
Ameda/Egnell milk bags, because they are nylon lined and very thick; but then
again I only recommend them for premies or ill babies that need every last
molecule of fat.  One suggestion for self-defrosting freezers is that the mom
store the bags of milk in a bowl of ice to prevent the milk from warming and
spoiling during the defrost cycle.  My moms have found this useful.
Catherine Watson Genna, IBCLC