Pillager:
You could always use a 9V-battery for your UNO. Just bend the Vin-pin from the shield aside or cut it off. That way you have 2 separate sources for powering the UNO (9V-battery connected to the UNO-board) and the shield (your 6V-battery pack connected to the shield board).Greetings
Tom
The motorshield I have has terminals for external power and a jumper to chose how to power your motors, via arduino or via the terminals. But I have the a similar problem, when I power the shield via an external battery the battery cuts power as I reverse both motors at the same time. Though I can reverse them one at a time without issue