FTTC download speeds wandering about a bit

[#1]

FTTC setup, has been running fine since installation in late Dec. Download speeds typically 70+ with the occasional drop down to the 50's or 60's at weekend evenings. 
This last week it's been very variable, and rarely above 60Mb/s.  Upload seems fine.  The BT report (run this evening around 7pm) shows:
1. Best Effort Test: -provides background information.
Download Speed
52.92 Mbps

0 Mbps 77.44 Mbps
Max Achievable Speed
Download speed achieved during the test was - 52.92 Mbps
For your connection, the acceptable range of speeds is 60 Mbps-77.44 Mbps .
Additional Information:
IP Profile for your line is - 77.44 Mbps
2. Upstream Test: -provides background information.
Upload Speed
2.66 Mbps

0 Mbps 20 Mbps
Max Achievable Speed
Upload speed achieved during the test was - 2.66Mbps
Additional Information:
Upstream Rate IP profile on your line is - 20 Mbps

This test was not conclusive and further testing is required.This might be useful for your Broadband Service Provider to investigate the fault.
[I don't think the upload rate is right on the above test - OOKLA is giving me typically near 18Mb/s]
The Plusnet test gives:
Estimated line speed:
There's no speed estimate currently held on your account.
Current line speed:
78 Mb

I've run the speedtest.net test a few times over the last few days, results are:
TEST_DATE TIME_ZONE DOWNLOAD_MEGABITS UPLOAD_MEGABITS LATENCY_MS
3/20/2014 10:16 AM GMT 24.27 18.87 16
3/20/2014 10:17 AM GMT 45.59 18.58 16
3/20/2014 10:18 AM GMT 27.7 18.23 15
3/20/2014 10:27 AM GMT 42.32 18.74 15
3/20/2014 11:38 AM GMT 48.64 17.46 16
3/20/2014 11:51 AM GMT 41.72 18.51 16
3/20/2014 1:21 PM GMT 76.92 15.62 26
3/20/2014 3:01 PM GMT 77.52 16.3 26
3/20/2014 8:28 PM GMT 48.12 14.39 17
3/20/2014 10:09 PM GMT 62.9 18.51 17
3/21/2014 6:12 PM GMT 62.35 18.29 17
3/21/2014 10:31 PM GMT 21.14 18.74 16
3/21/2014 10:33 PM GMT 58.04 16.15 16
3/23/2014 4:28 PM GMT 47.62 18.8 15
3/23/2014 5:33 PM GMT 53.51 18.27 15
3/23/2014 8:08 PM GMT 40.41 17.95 15
3/23/2014 9:15 PM GMT 31.03 17.69 31
3/23/2014 9:30 PM GMT 27.72 17.44 32
3/23/2014 9:51 PM GMT 50.44 14.48 48
3/23/2014 9:52 PM GMT 34.81 17.86 17
3/23/2014 9:54 PM GMT 39.54 18.38 29
3/23/2014 10:10 PM GMT 46.73 18.62 15
3/23/2014 10:22 PM GMT 53.96 17.57 19
3/23/2014 11:06 PM GMT 65.78 18.45 18
3/24/2014 7:41 AM GMT 49.52 17.61 25
3/24/2014 7:42 AM GMT 48.74 15.45 18
3/24/2014 9:29 AM GMT 48.9 18.77 15
3/24/2014 10:23 AM GMT 65.44 18.03 18
3/24/2014 12:26 PM GMT 63.45 17.18 25
3/24/2014 2:34 PM GMT 61.75 17.16 25
3/24/2014 2:35 PM GMT 69.22 17.63 18
3/24/2014 5:53 PM GMT 53.88 18.67 17
3/24/2014 11:31 PM GMT 60.02 18.65 16
3/26/2014 4:09 PM GMT 47.63 18.76 15
3/26/2014 5:22 PM GMT 48.93 18.62 17
3/26/2014 6:33 PM GMT 28.95 16.75 25
3/26/2014 6:34 PM GMT 60.73 18.7 17
3/26/2014 7:49 PM GMT 49.97 6.39 15
I wonder if this is a symptom of something not quite right?
Thanks for any inputs. 

[#2]

Hi JPN,
The speed tests that you've completed does dhow quite erratic speeds, more so on the upstream.
Would you be able to complete the TAP3 Test so we can get this investigated for you?

[#3]

I'd be happy to.  Could you explain where/how/what the TAP3 test is?
Many thanks

[#4]

Once you've completed the BT Speedtester at http://speedtest.btwholesale.com including 'Further Diagnostics' you should be given instructions on how to carry out the TAP3 Test.
The 'TAP3 Test' tests your line with test details in your router in a bid to try and identify which part of the network could be causing the issue. If the TAP3 Test fails/errors, can you attach a screenshot of the page?

[#5]

I've now run the BT test again a couple of times.  Now it's saying that download is around 73, [while the speedtest.net is showing 25!] It does not give me the option to run a TAP3 test.
Results Image not loaded

1. Best Effort Test: -provides background information.
Download Speed
73.29 Mbps

0 Mbps 77.44 Mbps
Max Achievable Speed
Download speedachieved during the test was - 73.29 Mbps
For your connection, the acceptable range of speedsis 60 Mbps-77.44 Mbps .
Additional Information:
IP Profile for your line is - 77.44 Mbps
2. Upstream Test: -provides background information.
Upload Speed
11.71 Mbps

0 Mbps 20 Mbps
Max Achievable Speed
Upload speed achieved during the test was - 11.71Mbps
Additional Information:
Upstream Rate IP profile on your line is - 20 Mbps

We were unable to identify any performance problem with your service at this time.
It is possible that any problem you are currently, or had previously experienced may have been caused by traffic congestion on the Internet or by the server you were accessing responding slowly.
If you continue to encounter a problem with a specific server, please contact the administrator of that server in the first instance.

[#6]

Hmm - your speeds are back to around normal. The TAP 3 Test will be offered if the initial test results are low and BT recognise that they are too low for your line.
If your speed drop again, you should be offered a TAP 3.