A few potential sources of problems:
1) Do you always buy milk from the same store? Perhaps the store employees are not very vigilant about getting it off the hot loading dock and into the coolers, shortening the shelf life considerably.
2) When you buy the milk, do you pick it up first or last during a shopping trip? Could you be letting it warm up enough in the shopping cart to let the bacteria start growing?
3) What about on the way home? Is it sitting in the hot trunk of the car for a while? I try to keep the milk cold on hot summer days by keeping all the cold/frozen goods together in the same bag(s). Pack it with the frozen peas and ice cream, and put it on the floor of the car interior rather than the trunk so it gets the air conditioning you're getting.
4) Have you thoroughly disinfected your fridge since discovering this persistent problem? You might have one of the bacteria that sours milk growing in a nice culture in there somewhere (have much yogurt with active cultures?) that's getting into open milk containers (doesn't explain the sealed container issue though). I'd suggest pulling everything out and cleaning the entire interior of the fridge with a bleach solution, rinse thoroughly and see if the problem goes away. Keep yogurts and cheeses in sealed bags or containers to prevent cross-contamination.
5) As suggested above, toss a thermometer into the fridge and check that the temperature setting is sufficiently cold for the milk. Also, check the door seal, especially if it's an older fridge. If there's a leak near where you keep the milk (probably the top shelf or door), that part of the fridge may not get cold enough, while stuff down in crisper drawers is freezing (that happened to me once with a leaky door seal...fridge kept running and cooling the bottom, but the top shelf was still a bit on the warm side due to the leaky door seal).