Its possible you have a line fault. I am assuming you are using Ethernet cable to connect your computers to the router. If Wi-fi, you could be getting interference from some source.
I suggest trying the troubleshooting guides on the PlusNet site as a first step. To try and see if its an internal issue. If you still get issues after doing the troubleshooting especially after being connected to the master socket then report as a fault so it can be investigated.