Speed has dropped

[#1]

I'm sure this is a common topic, but I thought I'd post it anyway. 
When we had our 76MB Plusnet service set up (May 2013), we got about 60MB down and 10MB up. We knew we wouldn't get the full 76MB down, but that was ok. However, while the service has been great for a long time, suddenly, our download speeds have halved to just 30MB this last few months. At best, sometimes 38MB. But since 38MB is the lower package than what we're paying for, I thought I should ask here.
Could someone help with this issue?
Thanks,
Adam.

[#2]

With such a dramatic drop I woiuld log a fault.

[#3]

Hi ImdurC, welcome to the forum 
I've had a quick look at your circuit and it does seem as though the physical speed on the line has dropped. 
I would advise running through the fault checker on the Member Centre which will raise a fault report through to our faults team to investigate.
Thanks

[#4]

Thanks for that Andy. Unfortunately, I already did and haven't heard anything back. I could try again, I suppose.
EDIT: I've just setup a ticket. Hopefully, it goes through.

[#5]

I've checked your account, nothing immediately shows any problems. Our faults team are due to pick up the fault ticket shortly. They'll do a thorough test on the line and update you.

[#6]

I look forward to hearing back. If the service can't deliver what it used to, we'll have to cancel the current plan. But then, I'm really curious as to why it's half the speed it used to be.

[#7]

If nothing is wrong with the line / cab / exchange or anything else with the BTW / BTO part of things and everything is fine at your end then one reason to your speed loss could be cross talk but I dont know with it dropping by half if it can be contributed to all of that, but there are others on here like WWWombat who know far more about that kind of stuff.
   

[#8]

FTTC has a drawback - it can suffer from crosstalk, which is the noise from other subscriber's modems. This can have an effect on your sync speeds over time - with you experiencing - at random - small increases in noise, and small decreases in speed when a new sync occurs. Less visibly, your line will also increase a gradual increase in errors.
Unfortunately, once this increase in errors crosses a threshold, it can trigger DLM into taking action. In the name of keeping quality, DLM will sacrifice some of your speed and latency - somewhere between 10% and 20% of speed, and an extra 8ms on latency/ping times.
Altogether, you should expect speeds to decrease over time - but generally, they should still within the range from the BTW checker.
BT know about this kind of effect, so don't generally accept a fault for investigation until speeds drop well below the expected range. A very large speed drop, in one go, is an exception, and will be investigated. IIRC, "large" means > 25% or so.
Note that everything I said is about sync speeds, and not about the throughput speed (especially at peak times). if your sync speed is still high, but you are seeing a reduction in throughput at peak times, then your problem is something else - probably a sign of congestion. There is a long thread on this topic in this thread.
However, Tony indicates that the speed reduction is on the line, so your problem seems to lie there - in either a fault, or in the gradual accumulation of crosstalk, or both.