I'd suggest a full audit of you operation, if this problem is causing failure and loss of product.
Do you sell your cheese? What grade is your dairy inspected for? Regardless, I recommend you try and meet grade A standards.
Examine every step of the operation, and find anything that MIGHT cause a problem. Fix it, and try again. If such an attempt still doesn't fix the problem, then I'd say you need mineral supplements for your cattle, they're not getting something they need. (family in the feed business for generations, I know a thing or 2 about what mineral deficiencies can do to dairy)
Have your milk chemically tested -not just the mandatory population tests, but chemical tests. might cost money, but it can tell you just what you need to be giving your cows.
Grass fed is great and I'm all for it, but cows are not native to this continent and their digestion is not balanced to strictly forage off of our grasses -that and you are limited what and where they can go. So bottom line is, they need supplemental minerals to keep things balanced.
Also, probably you know this, but keep them off of the early morning grass. You don't want them eating the dew, this can cause digestion trouble as well, leading the chemical imbalances in the milk