I'm about to take my ZyXEL off line to see if another modem/router will repair the damage done by the DLM.
Will not be much to log when off line but I'll give it a go. If you've got a Raspberry Pi and wish to log the memory level yourself I'll post the logging scrip
I notice in the firmware V1.00(AAKL.7)C0.zip file there's a .rom file, this appears to be a backup/restore conf file. I plan on reloading this firmware and seeing if the .rom file will restore the default settings. Doing a factory reset doesn't completely renew the conf setting so there may be something corrupt there.