XP: Frozen Milk Going Sour

We recently discovered that my frozen milk is prematurely souring. I thawed some yesterday that is a month old and it smelled and tasted horrible. I tried thawing a few more bags and they were also sour. One was only a couple weeks old. Then one bag that was 3 days old was fine. I also have some milk in the fridge that was pumped 36 hours ago and it is starting to acquire a smell.

I remember hearing about potentially having too much lipase in the milk, and have researched that issue and read about it on kellymom.  But from what I've read, it seems like this would make the milk taste soapy and not sour. I've also read that it could be due to water contamination where the milk is being oxidized by metal ions in tap water. I try to minimize any water on pump parts and storage containers, but read I can rinse them with distilled water to help with this issue. 

Does anyone have any experience with this? I feel so frustrated because my entire freezer stash is now pretty much down the drain. I'm not fond of the idea of scalding milk to deactivate the lipase because it also damages other components of the milk. I don't even know if it IS a lipase issue. Just don't know where to start.  

The milk is stored in the back of the freezer and fridge, so I don't think it's a temperature issue.