I have already contacted Support about this issue. Connection drops seemingly randomly. So far I have tried:
6 ADSL filters
3 cables from router to socket
plugging directly into inside of BT socket (i.e. taken front plate off)
completely removing phone from BT socket and immediate vicinity of router
And yet it still drops. I came home a few days ago, no-one in the house, nothing connected and the light was red. It dropped again this evening (I noticed at 6pm) - no-one was using anything connected to it. The router is already plugged directly into the box, I unplugged the phone, I turned the router off and on again. It came back at about 6.25pm.
PlusNet can only suggest sending a BT engineer - sorry I don't trust them (previous experience). Incidentally we have seen a BT engineer 3 times over the last 2 months at the green junction box 50 yards from our house. Coincidence?
As you have narrowed down possible problems inside the house, then it would appear the fault is in the line between you and the exchange, hence why a BT engineer is required.
Whether you trust them or not, there is no one else that can check those wires out. It could be water in a junction box somewhere, the cable rubbing on a tree, a loose connection, or any number of things, so best let them get on and find it.