I've have a similar problem on and off over the years. We milk 4 cows and several goats everyday. All my cheeses are raw milk cheeses. I never have this problem in the winter time, only in summer, when it gets dry and dusty here in California. I think it my problem is yeast, at least in my case, the curd comes out spongy, full of small holes, pig food, in other words. I now have come to associate this curd problem with dust in the milk parlor. I milk with a milking machine, an old fashioned one with a bucket and milk claw. I clean and sterilize it after each milking, and I used to think that was enough...but then I started getting these stuborn yeasts! Very frustrating. I tried bleaching everything, spraying down my humble milk parlor with bleach spray, scouring everything comming into contact with my milk, I used really hot water, soaps and sanitizers, that helped of course, but did not entirely eliminate the problem. After struggling with this problem off and on for a couple of years, I finally figured that airborn yeast must be contaminating my milk. I finnaly realized it must be traveling up into the teat cups of my machine on airborn "dust" particles, while my cleaned milking machine just sat there in between milkings... so I started putting a big black plastic bag over the milking unit after cleaning it... and guess what... I've had no more problems with spongy curd (knock on wood) ever since I started doing that! Such a simple solution that really worked for me.