My cable config is just main line to property in brown box connected to the external cable (10metres) around side of property to isolator, then connected to modem, I have no splitters on the line as I only use their broadband, no TV etc, etc.
Maybe the Isolator ? I have no others to test unfortunately.
When kids have gone to bed, I'll undo the isolator and test without it.
Edited by deleted (Tue 22-Mar-16 18:52:53)