Hi richard,
there might be a loose contact/cold joint forming in or around your CPU/Memory circuit.
In that case, it might be the prelude to a sudden death.
First off, i’d recommend immediate and complete backup of all data that you might loose if you need to reset or if the phone stops working.
Also, it might be that the memory is getting full. It needs a bit of free space to function normally: if it’s at 99% used, it’s going to have problems that can culminate in a bricking.
So, my advice for now is :
- Backup
- Make space for memory: delete backed up photos, unused apps, etc.
- If there’s still a problem, you could try to reflow without waiting for the sudden death? i’m really not sure about this ; maybe wait if a real sudden death occurs