Arduino gets stuck: battery powered, wireless, dc motor drive
My guess is that your motors are drawing too much current and the voltage from your battery is dropping too low for the voltage regulator on the Arduino. Looking at the data sheet, the L293D will provide up to 600-mA for up to 2 motors at a time. That's up to 1.2 amps, a whole lot of current. I would suggest trying to power your arduino from a separate 9 volt battery as an experiment.