What you have described is identical to the problems I am having.
I rang the 50mb support line, who could not see anything wrong. Luckily at the time I called, the problem was bad, and downloading multiple times from the blueyonder demo games site showed 1 at 1.5MB/s and the other at <50kb/s.
The only thing that support could see (that I suggested) was that one of the 3 download power levels was slightly above 7.
So engineer came out yesterday to fit an attenuator? and now the levels are down to 0, similar to yours.
Its only been a day so far, so can't really tell if the problem is fixed or not, but honestly I'm not hopeful.
I know its difficult for support to identify and fix intermittent issues, but there are quite a few posts here and elsewhere describing exactly the same problem. That suggests to me, that there is a bigger fault somewhere that has not been identified yet
