After a few tests and observations, I decided to change the hashboard arrangement again.
Thus I have even more airflow in the middle.
Currently everything is running very stable at 625 mhz.
The 4th Hashboard is from Batch 2. To overclock this board even more will soon follow some soldering.
Currently I'm looking for the right inductors and the right transistors, etc.
If someone can help me with this, feel free to contact me.
Happy Mining to you ALL !!