Speed getting worse as time goes on.

[#1]

Hi All,
I upgraded to FTTC as soon as it became available on my exchange about 11 months ago. I was the first person on my cabinet the engineer said. At install my speed was 67 / 18 and I was more than happy.
As time has gone on I've noticed the speeds getting worse and worse. 67 became 63, became 58, became 49, became 43 and now I am seeing 38 / 10
I'm paying for the higher package but getting the lower package speeds. I have read up about cross talk and interference etc, but can it really affect me this much? If I downgrade to the 40meg package will I still get 36 do you think or am I likely to drop to 20 or something ridiculous?
I did attempt to raise a ticket a few months back when it was in the 50s but I got told is was in the expected range.
Really frustrated, Any ideas?
Thanks

[#2]

If you downgrade your downstream speeds (assuming your sync is already 40 mb/s or less) should not be affected but please be aware that the upstream speed on the current Plusnet lower FTTC package (Fibre Unlimited) is capped at 2mb/s.

[#3]

Thanks.
2!!!!!! ... what a joke.