Thanks all for the messages, I've tried with another one, 6.5V, and it's the same, it was rebooted 3.5hours ago. I don't know if it could be a kernel problem, I'm using also a M4V1 with SATA hat, and kernel 5 and works flawless (Up time: 27 days 1:15), so I think it is related to the PSU/cable...
My main problem here is I'm not good with electric stuff, meters etc, I have an standard meter, but I don't know even how to use it
Sorry for asking @NicoD with the usb meter, should be plug and see? if so, I think even I could make it.
I'm going to look for a good usb cable...
Thanks.