FTTC Speed Issue

[#1]

My FTTC cabinet is about 1.1km away and the aluminium cable is about 40 years old. Soon after I switched to fibre, OR replaced about half of the cable length with copper.. This put my download speed up to around 17 Mbps. Recently I had a number of disconnections and the speed dropped to around 14 Mbps. The disconnections appeared to be due to the router re-booting and I decided that the 582n was faulty and bought a HH5. Since then I have no drops in connection (except for BT admin activity on the HH5) and my line was syncing solidly at 18.4 Mbps (download speed 17.25 Mbps)  with a max data rate around 21.5 Mbps. I don't know for sure whether there was a line problem, a router problem, or both.
This was fine for about 3 weeks but then the noise margin dropped to around 4dB. Although there were no disconnections I am aware of, the sync speed fell to 15500 Mbps and the max data rate fell to 18.4 Mbps. Since then the max data rate has returned to approaching 22Mbps, the noise margin to 6.6dB. There has been no upward change in the sync speed after around 6 days of improved figures. ( I have reconnected the PPPoE session twice to see whether that triggered anything.)
Do I need to wait longer to see if the DLM increases the speed?
Is there an underlying problem that can be investigated to stop the loss of speed?
Am I stuck with a line that generally appears solid but will suffer from occasional problems until BT saves up enough to replace the rest of the aluminium cable?
I know it's only 3 Mbps but that is about 20% of the line speed! Any thoughts appreciated.
3. Firmware version:Software version 4.7.5.1.83.8.173.1.6 (Type A) Last updated 16/05/14
4. Board version:BT Hub 5A
5. VDSL uptime:8 days, 10:30:14
6. Data rate:1702 / 15534
7. Maximum data rate:1786 / 22120
8. Noise margin:6.2 / 6.7
9. Line attenuation:0.0 / 32.6
10. Signal attenuation:0.0 / 27.5

[#2]

Hi jsm51,
After testing your line and checking your speed profile and line check, I can see that you're sat at about the correct speed for your line at present(small tweak required of less that .5mbps). You may well see an incremental increase over time however you're sync rate is currently.
I have made the minor adjustment for you now and you should see a small increase soon.

Thanks,
Adam C

[#3]

Thanks, Adam,
Typically, just after I posted, the sync speed increased by about 1meg to 16.245 Mbps - still a good 2Mbps short of where it has been. The tweak to the Plusnet profile will help but I am still wondering why (out of technical interest if nothing else) a line should show periodic drops in performance which then come back relatively slowly.
5. VDSL uptime:0 days, 01:20:54
6. Data rate:1692 / 16245
7. Maximum data rate:1781 / 18902
8. Noise margin:6.3 / 6.0
9. Line attenuation:0.0 / 32.6
10. Signal attenuation:0.0 / 27.4

[#4]

Hi jsm51,
This is normally the work of DLM, more information on this can be found here.
Normally this will be the result of a jump in interference and the DLM will lower the speed in order to keep the connection. If this were a fixed service a jump in interference could well mean a loss of connection so the DLM is a handy feature to combat this.
It may be worth recording when this happens to see if it's a regular thing and happens when something electrical is used in the home or maybe plugged into the line. Given that you seem to see these drops infrequently it could well be hard to diagnose.
On the positive side, the DLM will normally monitor the line and keep nudging the speed back up as time goes on with the line being stable so you should be beak to full speed at some point over the next few weeks.
Thanks,
Adam C