Correction - I have taken two additional steps to try and confirm this. If I use a relay and switch the VDC to the USB port on and off it works fine. If I remove the VDC from the USB port by just taking the +wire off on the power supply it does not like that and reboots possibly because it is NOT taken away cleanly.

@Nagarjuna-Reddy use logread command then use grep to filter.

@David-Kerr thanks - i'd considered the hardware option but I don't want to damage my board I will have a poke around the power-dock-2 script and see if i can figure something out from that.

There is a burst of energy required when the WiFi transceiver begins operation. You, as many of us, discovered the magic moment that it happens. Not knowing your circuit I can only speculate. As such, you may get past the burst point with a "large" amount of capacitance across the power input and ground. Using an expansion dock (which you are not) I'm using a 470uF cap just downstream of a 5V regulator just at the +5/gnd dock inputs (there's actually a small cap just upstream of the regulator as well). This has apparently "solved" my problem, however occasional brownouts occur putting the system in the dreaded 3 byte/4 byte reboot issue (another big topic on the forum). Try a cap and see if it helps. Additionally reduce inductance (wire length) between the power supply and O2. If those don't help, you'll need a new power supply. --Bill

In order to understand what is going on, you should check the serial console during boot.