Update:
Well the engineer has been and gone and everything works now.
He said that 55dbmv upstream was too high a level and with the splitter on it would push the level even higher and that was why I was losing connectivity. The signal could have been weakened by lots of factors but his best guess was that someone had been in the green box and moved me around.
The issue was resolved by swapping out the old Motorola modem for a black Virgin modem. Apparently they cope much better with weaker signals, and I saw a corresponding power drop from 55 to 40 as proof. The connection is rock solid now
