BT Inifinity getting slower and slower

Question

Please forgive the slightly long post...
I had BT Infinity installed nearly a year ago and was initially promised (based on line checks) a 28 Mbps connection. This was achieved for a week or two until the speed started dropping and finally stabilised around 19 Mbps. I called BT several times and each time the helpdesk staff would perform some line tests (which came back with no faults) and reset my IP profile, after which speeds would improve for a week or two before dropping again.
In the midst of this, I was contacted by BT and offered Infinity Option 2 with a promised speed (again, based on line tests) of around 68 Mbps. So I agreed and my account was upgraded.
As before, the connection started off at around 46 Mbps but, after a few weeks, stabilised at around 35 - 38 Mbps where it remained for several months and I was happy enough (although I would've preferred the promised speed). The improved speed meant I started happily watching TV and movies on services like Netflix and Sky Anytime+.
A few weeks ago, however, I noticed that I started getting regular pauses and extended buffering when watching streamed content. So I ran the speedtest on www.speedtester.bt.com and it came back with a speed of 16 Mbps. I tried again at different times of the day over the next few days and always got speeds of between 15 Mbps and 19 Mbps. In between tests I tried rebooting both the router and the modem.
A few nights ago I lost connection completely for an hour or so, but it came back eventually.
This morning I noticed that my general web browsing was slower and performed another test. This time my download speed was assessed at 9 Mbps and I was presented with a request to change my broadband username and perform an additional test step, which I attempted to do. Unfortunately, although I could see the router had connected correctly with the new username, the tester kept saying that it couldn't perform a test at this time and I should try again later.
I've been retrying the test over the last couple of hours and I keep getting the same error message. The error persists even if I restart my browser and try the test again from the beginning, either with my existing broadband username or the test one.
So I'm back on my 9 Mbps connection for now and feeling somewhat hacked off, both at paying for a 68 Mbps service that I'm not receiving and because the testing tools provided by BT seem somewhat unreliable.
Please could someone suggest what might be wrong with my connection that causes the speed to degrade over time? I've read the threads about BT's systems revising the IP profile down, and realise this might be the cause, but it seems ridiculous to reduce the speed of a connection (that's been happily working at 35 Mbps+ for months) and never try to increase it again, unless I phone them of course.
I can confirm that the modem is plugged into the master socket, the computer is connected directly to the HomeHub using Cat-5e cable and that I'm not running any other software or using any other Internet connected equipment whilst performing the tests. I've done a quiet line test (as had been suggested by an engineer in the past) and the line sounds almost silent to me. I would like to check the SNR, etc. of the connection but this doesn't show up on the HomeHub 3 as it did when I had a HomeHub 2.
Any suggestions?
Thanks,
James

Answer 1

A very helpful engineer arrived on Friday and did a full test of my line, checked all the joints back to the cabinet, the cabinet itself and the connection to the Exchange and couldn't find any faults. He then noticed that I had an older revision of the VDSL modem which apparently should have been replaced due to known problems. The modem was replaced, my profile reset and my connection was back up to 50 Mbps out of a possible 55 Mbps, where it remained for the rest of the evening. I was very happy.
Unfortunately, the next morning I awoke to find I had no broadband connection at all. It remained stubbornly unavailable for at least an hour, after which point I had to go out. It was back up when I got home.
Since then, the connection has been down for varying periods, between ten minutes and an hour or two, once or twice a day (it could be more but I'm out at work) almost every day. Performing my first speed test since the engineer's visit shows that, as of this morning, my profile has already been reduced to 33 Mbps, of which I'm receiving 30 Mbps. I have not disconnected or reset the modem or the router since the engineer's visit.
Whilst I appreciate that this morning's speed is within standard service parameters, I'm worried that I'm going to continue seeing the speed reduce over the next few weeks (as before) and that it's also nowhere near the speed I was told my line should be able to support. On top of this, the regular interruptions to the service are also extremely annoying.