It sounds like you may have a component failing on the board, probably in the power supply circuits. I would say to check the electrolytic capacitors, since a failing cap could cause this behavior, but looking at the schematic, it doesn't seem to have any. There are a few 10uF caps that may be worth checking. Other than that, may need to probe with an oscilloscope or meter and see what is happening when it fails.
Since more commonly, failures are mechanical, I would also clean and re-seat all connectors, and see if any of them made a difference.
Schematic:
And BTW, your original post, and your repeat of it here has an error: you say you reflashed with later firmware, but you gave the same old date code for the new firmware, so we have no idea what firmware you loaded. And most on this list know firmware by its version number rather than build date code. But that is probably not relevant, if the unit is suddenly powering down in DFU mode. The DFU mode bootloader is in ROM on the STM32 microprocessor, you can't brick it with a firmware download - so if it is spontaneously powering off in DFU mode, it must be something in the hardware.
Good luck, Stan
