VR612 power supply problem

Hi I have a VR612 which is power supplied using a 5V 4A PSU plugged on the VRBS1.
Everything is working fine when the arduino mega is connected to PC via USB.
When no usb cable is plugged on the arduino, allbot is rebooting after few moves,
few moves then rebooting again and again.

Do you know what is the problem and how to fix it?
thank you