Power to arduino

i have a wireless robot that has 12 motors.. the motor driver board i am using supplies a regulated 5v to the 5v pin of the arduino.. however when all 12 motors are switched on the arduino shuts off. so i put a 9v battery onto the Vcing in pin.. but still the problem persists... however connecting the arduino to the lap, the robot works fine..

What could be the problem?

Don't try to derive the 5V from the motor battery pack at all, this is obviously drooping
under load. Adding to Vin won't cure this unless you remove the connection to the Vcc
pin which is dragging down the voltage.

Separate power, easy life...

Are you powering the motors from the Arduino 5V line? If so, that's your problem. There's not enough current available. You'll need to provide external power to the motors. Just don't forget to share the grounds.

i am providing power to the motors seperately.. But the motor driver i use needs Vcc and GND. I connected the 5v pin of the arduino to the Vcc of the motor driver. Now 5v is coming back through this and powering up the arduino

Double check your wiring. It sounds like you're getting a short circuit when you give your board power with an external power supply.