What I ment about the mixing wasn't saying your radio was broken, just setup wrong. Thats why I wanted you to use channels that are never mixed in the radio(eg throttle is never mixed with anything as far as I know).
I'm not sure if we are just not communicating properly.

When switch 4 is ON it is in mixing mode(eg Both motors will move forward/backwarsds when you move the stick on one channel, and both motors will move opposite of each other when you move the stick on another channel(its mixed so it's not exactly that but it's close).
When switch 4 is off you are in tank mode(one channel controls one motor and the other channel controls the other motor). This is why I thought you may have mixing setup on the Radio(not mixing mode on the roboclaw) because when using mixing on a radio one control(eg the elevator axis of one of the sticks) can control 2 channels at the same time. This is why you could have one stick direction(left/right) control one channel(running just one motor), but have another stick direction(up/down) control 2 channels(causing both motors to run) when in tank mode on the roboclaw. If you had mixing on the radio setup and mixing mode on the roboclaw setup you could have even weirder interactions.
I think the fastest turn around will be to send it in and we'll take a look at it(since we have the same radio to test with). We'll replace it if it's a hardware problem or update it if it's a software problem.