My Pi shorts every time I run my program. [PiFace]

Question

Every time I press a button on the wiimote controlling my robot (Rover 5 with PiFace), the pi restarts. My code is as follows:

import cwiid
import time
import pifacedigitalio as pfio

pfio.init()

button_delay = 0.1

print 'Press 1 + 2 on your Wii Remote now ...'
time.sleep(1)

# Try to connect to the Wiimote & quit if not found
try:
  wii=cwiid.Wiimote()
except RuntimeError:
  print "Can't connect to Wiimote"
  quit()

print 'Wiimote connected'
wii.rpt_mode = cwiid.RPT_BTN

while True:
  buttons = wii.state['buttons']
  if (buttons & cwiid.BTN_UP):
    #Forwards
    time.sleep(button_delay)
    pfio.digital_write(2, 1)
    pfio.digital_write(3, 1)

  elif (buttons & cwiid.BTN_DOWN):
    #Backwards
    time.sleep(button_delay)  
    pfio.digital_write(4, 1)
    pfio.digital_write(5, 1)

  elif (buttons & cwiid.BTN_LEFT):
    #Left
    time.sleep(button_delay)         
    pfio.digital_write(2, 1)
    pfio.digital_write(5, 1)

  elif(buttons & cwiid.BTN_RIGHT):
    #Right
    time.sleep(button_delay)          
    pfio.digital_write(3, 1)
    pfio.digital_write(4, 1)

  else:
    pfio.digital_write(2, 0)
    pfio.digital_write(3, 0)
    pfio.digital_write(4, 0)
    pfio.digital_write(5, 0)


#press button A to stop all motors
  if (buttons & cwiid.BTN_A):
    time.sleep(button_delay)          
    for i in pins:
      pfio.digital_write(2, 0)
      pfio.digital_write(3, 0)
      pfio.digital_write(4, 0)
      pfio.digital_write(5, 0)

Answer 1

The power supply you are using to plug the Raspberry Pi to the mains is probably unable to deliver the required power.

You don't say where in the world you are. In the UK, one of these should do the trick.