Dagu ComMotion motor controller shield

I've just started working on a project with the Dagu ComMotion shield and I'm hitting a few problems.

I've fixed one issue in the ComMotion firmware where the motor controllers wouldn't work properly in non-encoded mode, see:

I'm now getting a problem where everything works fine when the motors are running in reverse, but the shield will reboot as soon as any motor runs forward. It looks like setting the direction output high for any motor causes the problem. At this point I don't have any motors actually connected, I'm just looking at the LEDs on the shield. The shield and Arduino Uno are being run from an 8V Li-ion pack.

Any ideas would be much appreciated! The shield user guide is here: