We have found the problem.

It looks like the old motherboard can not support two USB 3.0 devices on full load.
My husband recently got a new 7.1 Headset, which seems to need more power while in a game that supports that.
Why it only deactivates one key is beyond me, but at least we now know the answer.

We work around this for now by connecting the mouse to USB 2.0, and will update the motherboard in time.


Hope this helps if someone is having a similar problem.

- Thanks again and issue is now resolved. -