If the cable has been in place for a while, then it it is very possible that there is a break in the outer cover letting in water, as a general rule the long lenght of the the cable will be fine its where the stain on the cable is taken that you get breaks in the insulation, so the point at which it attaches to the house or the pole, or the point where the cable enters the ground and goes to the local box.
The only other thing is has there been any work localy to the roads, as utilities are not unknown to damage underground cables and say nowt to the phone companies and just hide the evidence, so if the problem has only just started and you can link it to road works then it may be possible that has happened. Either way its down to the service provider to sort out.
You might want to try checking the quality of voice calls at the same time as the internet is poor, if voice quality drops at the same time it is very likley to be a line fault.