This morning one of my Zyxel VMG-1312-B10A rebooted at 8am after around 14 days uptime in bridge mode. It has been timing out in DSLStats for telnet for quite a while and seems sluggish on the web GUI. So I have replaced it with another 1312A which seems to have cured the problem of telnet timing out. It's a shame because the one I have replaced had the highest sync out of all of them. Think it's on it's way out?