Why yoghurt comes out different every time? When I make yoghurt at home the result is different every time even though I follow exactly the same procedure:


Bring a gallon of milk to 85°C
Cool to 43°C
Add yoghurt culture from the previous time
Keep it at 43°C for 2-3 hours or until ready
Cool it to refrigerator temperature


Milk is always of the same kind from the same store (Costco). I use thermometer and keep the temperature with 1°C precision.

Sometimes it is extremely creamy, soft, homogeneous, and almost sweet, and sometimes a lot of whey forms on top and it becomes very sour.

Obviously I like the the former outcome rather than latter one.

I can't figure out what is wrong, why it is different every time?

I also noticed that when I start with the store-purchased culture the first batch is likely good (creamy).
The explanation is easy, the correction isn't. 

Bacterial growth is an exponential process, but with all the environmental variables, you get even more trouble than a simple exponential. Tiny changes in the initial conditions can result in completely different end states. What you are seeing in the "sour" state is either too much bacterial activity for your taste, or activity from the wrong kind of bacteria - thermophilic cultures produce a more sour yogurt than mesophilic ones. 

Note that the low-sourness state is not obviously the preferable one, and your recipe may well have been geared towards producing the sour state. 

The way to getting to the exact result every time is to be more consistent in the initial conditions. A laboratory works with much higher precision than what you describe, not only in temperature but also in selecting the culture type and the culture density and reinoculating with pure culture. 

Your best chance here would be to try using lower temperatures and see if it gets better. If you can find a temperature in which you can get the creamy style yogurt consistently, great. If not, you might need to find other ways to ensure precision, such as using a commercial culture and not reusing your own.