Just to quote myself and state the results, the firmware upgrade has helped with stability quite a bit. Bumped up the power to '6' last week and maintained 1.4TH/s since then (no reboots, yay!). No cores have gone offline, and there's been less fluctuation with hashrate (both from the device 'status' page and from the pool). I'll try increasing the power a bit more over the next few days and see how it holds up.