Line Follower: can my code be optimized?

I'm able to rotate the wheels forward at max speed (PWM 255), backwards,.. anything I can imagine except for two wheels backwards at the same time. It just stops and resets.

As for power, I'm putting 10,5V on the Vin of my board as well on the driver board I got.

The driver board is this one :
http://www.lctech-inc.com/Hardware/Detail.aspx?id=155c73e2-e972-473f-913d-ec443dbef0be

In my loop() , I can write:

Motor(120,0);
delay(500);
Motor(-120,0);
delay(500);

It works, the wheels rotate in order changing direction over and over again without the board resetting , but when I try:

Motor(-120,-120);

or

Motor(-120,120);
delay(500);
Motor(120,-120);
delay(500);

The whole thing resets :frowning:
Thanks for replying by the way! I appreciate it alot