Are you by chance using the included ethernet cable with the ferrite core? We had this problem for several month because we didn't use the included cable because it was to short. The 8831 would lose registration, dead or locked up screen or really bad static/feedback/squealing sound on the audio. A reboot correct the issue for about a week, sometime a bit longer.
After a bitch session with our vendor and a demand for refund on 30 of those thing, it turns out, there's some use/function to those ferrite core after-all. At our vendor request via Cisco TAC, we removed the ferrite core/bead from the original cable and put it on our own cable and no more issues with the 8831.
Try that if you haven't - good luck. At 800.00 a pop, I would think they would work better...