If you are using the Pololu motor controller, make sure it's reset pin is held high. If it dips to 0V, it will reset the controller to its inital state (motors off).
If you are using the Pololu Low voltage Dual serial controller:
The green leds on the controller should also be on for forward settings, see if they are still on when the problem occurs.
It looks like you have enough of a delay after the motor serial commands.
--John
Bookmarks