When logic fails, blame the natural World!
It appears one of our cats had chewed a patch lead. I found six or seven teeth marks in it, and suspect this to be the cause.
This patch from the 99 is only exposed for about one foot accross a base board and is hidden/protected for the rest of the length.
Wierd as a power cycle of the 99 would result in a stable connection (static IP). I suspect only when a decent level of traffic flowed through, would it trip up.
All appears to be fine, thanks!