It may be a hardware overheating issue. Did you install windfarm_core and windfarm_fcu_controls to control the fans? -Stan Johnson
I've enabled them since my upgrade to sid at the end of December but I mentioned an other issue here yesterday : https://lists.debian.org/debian-powerpc/2020/01/msg00062.html
I would really be surprised if it is an overheating issue and more if it is a hardware problem (and I would be saddened...) . As I said, it shutdowns randomly and sometimes very early after boot. With no intensive task. It has never occurred during Sid's installation process (I tried several times).
With Wheezy, I doesn't occur either. And I do intensive tasks with it : audio processing with ardour, parallel builds with both cpus and so on. The temperature of my cpu does only increase from 55°C to 67°C doing that. My SSD is 37°C and my HD is 44°C now.
I will monitor those temperatures with sid.As John Paul suggests, I will try different kernels. First, can I try Jessie's kernel in Sid or will it be a problem with my ext4 root partition ? I'll try to build more recent kernels after that.
Thanks for your kind answers. Bertrand