Modem Problem?

Hi, I am a home pc user connected via ethernet cable to the internet via an NBC-VAC modem provided by my ISP. My modem is connected to the house ringmain with D-Link PowerLine AV 500 Passthrough Adapter as is my PC. These have worked well and continue to do so. My contract also includes unlimited fixed line telephone calls.
Recently I have been experiencing a significant drop in my internet speed and sometimes total loss of telephone connection. At times the problem seems to correct itself for a short while, but soon reccurs. Surfing the net is extremely slow and sometimes it takes 30-40 seconds to obtain a dial tone when I need to use the phone. Also during calls the recipient often loses my speech or the call simply drops altogether.
I have rebooted the modem many times, removed all wires and plugs etc but all to no avail. My ISP seems to find nothing wrong at its end and I am wondering is the fault is in the modem itself or the telephone line into the house. Repairs to the line leading into the house will fall to the ISP and I accept that any internal fault would be my responsibility.
Before proceeding further, does anyone experienced a similar problem a d can offer some advice.

I don't know where in the world you are but if you are in the UK then your main line box into your home will have a test socket behind the front panel that screws off. If you call your ISP they will ask you take that cover off and connect to your line via that socket. If things are still not working then the problem is down to them or your line provider and they should arrange for an engineer to call. Hopefully your line provider and your ISP are one and the same or you will be bounced back and forth between the two each blaming the other. If you are anywhere else I don't know how it works.