If you are using one of those wimpy small 9 volt batteries then you are probably just exceeding the current draw capacity of the battery. Those small 9 volt batteries should be outlawed for use with an arduino, they are just so expensive for the small amount of enegy they can supply. Get a proper battery source for the arduino. A quick check is to read the 9volt battery voltage while the 'bot' is in it's failed condition, If it's lower then around 7.5vdc the arduino board will just drop off due to the auto-voltage selection circuit.
Lefty