Mega 2560 with motor shield

Hi all,

I’m in a project to make a robot car wich follows black lines. For this, im using:
-2 6V DC motors
-4 colorsensors (TCS230 TCS3200 kleurherkenningsmodule - Ben's electronics)

-ultrasonic sensor(the one you can find anywhere)

-MOTOR SHIELD (L293D Motor shield Arduino Uno en Mega compatible)

I’m also using a LEGO NXT battery as power supply.

The problem is, after a few seconds the motors heavily take down, and stay still. After turning the aruino off and on again, it works for, again, a few seconds. We hear the motors running all the time, but with that the voltage takes down either and gets a huge resistance.
We tried:
-a new arduino board
-added 2 batteries serial
-replacing motors(several times!)
Changing the ports of the motors pinned in the motor shield.

It all doesn’t work, so I hope someone here can help me with this specific problem. Thanks!

Get a better motor shield. L293 (Darlington) based motor drivers are usable only with higher voltage and then with low current. Feel yourself how hot your driver chips get.

Thanks, I’ll buy another one. Wich one do you advice?