Wondering if you ever got this resolved. I am having the same problem. Port mirroring is set up correctly. If I go to "Power Saving" and hit the "apply" button, whether or not not power saving has already been enabled, port mirroring works for 21 seconds (21 pings) - then stops. if I hit the apply button again - mirroring works for another 21 seconds. Doesn't matter if power saving is on or off.
Mirroring was working fine for about a week after I got the box - then it started to exhibit the above behavior. Big hassle to send it back.