It seems like the usb controller used* *in Rspberry pi SOC (Broadcom BCM2835) has some serious issues , since it can only supply low amount of current to its USB devices (Approximately 140 mA). This will cause continues restart of RPI specially while connecting USB Network Card, USB Modem or GPS devices.
This may be a bug in USB driver, which has not fixed yet and may not be an easy task because :
(Ref: http://hardware.slashdot.org/story/12/08/24/2228251/serious-problems-with-us... )
1) Documentation on the Broadcom USB controller is proprietary and is not made openly available by the Raspberry Pi Foundation.
2) The USB core for the Broadcom SoC was bought from a 3rd party, and we are told that not even Broadcom has full documentation, nor understands the driver.
3) The Foundation has discovered that the controller and its driver expect realtime response from the ARM core, and if Linux's non-realtime scheduling doesn't respond in 1 ms, a split transaction USB event can be dropped. Not surprisingly, this occurs regularly and produces lost mouse clicks, stuck keyboard keys, etc..