Thanks Pipis, this clarifies for me, have been battling this for the last month, probably even more and it corresponds with calving season which will probably be sometime in September. Have tried pretty much everything else, I was usually heating up to 75^C for a minute or so and it always made delicious yoghurt (I use raw milk), not real greek style thick but with a bit of acidity and not slimy at all. Until about a couple of months ago. Unfortunately I opened new batch of culture at the same time so this was first to blame. I got a different culture from another source and it doesn't seem to be doing any better. Heating up to 95^C for 10 minutes makes very thick yoghurt but appears slime is still there, particularly in whatever little whey is left. I used to love it but am feeding it to the dogs now once it accumulates on the top (once I take some yoghurt out, I have 1 litre pot). Was thinking my 99% alcohol was not doing the sterilizing equipment properly or something but was also starting to suspect the milk.
Since our source of raw milk is drying the herd off for calving in a couple of weeks will see how we go with another source when I am able to find it since prohibition on raw milk here is becoming ludicrous. Booze and cigarettes in dairy on every corner but oh no, raw milk, you will die from it...
Glad I found the forum, I was starting to doubt my yoghurt making capabilities.