Variable Infinit speeds

Hello, I wonder if any BT guy can throw some light on the problem that I've been having - off and on - since May this year. Prior to May, I had BT Infinity 40/10, which worked satisfactorily for the greater part of the year or so that I had it.
I was then moved on to 80/20 and since then my speed can be anything between 0.89meg & 71meg: there is no consistency with regard to times when the speed is at one extreme or the other.
Despite much interaction with BT, the problem is still the same e.g. 09.30 speed 67meg/10.12 speed 14meg/12 noon speed 67meg/13.14 speed 22meg.
As I say, time of day has been completely eliminated - I have umpteen saved screenshots showing times and speeds.
I was finally contacted by a "senior" BT bod, who told me the issue must be with my PC, etc, as my IP profile had remained constant - at 69.8 approx - over the duration of my problem:. As I understood it, from him, this profile reading indicated that I had "had" full speed at my wall socket all the time: is this fact or bull?
BT sent a replacement HH3 - no difference - they have never replaced the modem
My installation is thus;
BT wall socket is 2 yards from my front door
Using BT supplied lead to modem - 2 yards approx.
Using an 18 inch ethernet lead from modem to HH3
Using 18 inch ethernet lead from HH3 to PC
"No" extension wiring at all, in the house; phone connected to the phone outlet on the BT wall socket.
Tried other cables - modem and ethernet.
PC
Intel 3GB core duo
4GB DDR2 - plenty to spare at all times
80GB hard drive - plenty to spare, 30GB approx
Microsoft Security Essentials A/V
No VPN or proxy usage
Any help gratefully received - hopefully by BT care staff!
Cheers, Les.

If the IP Profile is constant then it shows that the VDSL connection itself is fine, but that is perhaps a few hundred metres, the bigger chunk is the fibre that will be many many miles from cabinet through to where the sites you visit are hosted.

www.speedtester.bt.com what sort of speeds does that give?

The speed problem is it showing up on just speed tests, or are you seeing lots of very slow web sites too?

Hi Andrew, Thanks for the quick reply.
I use the site you suggested plus
TBB
Speedtest.net
and the BT Openreach recommended site http://speedtest.btwholesale.com/
The results from any of these sites seem pretty consistent i.e. very good/poor or very bad!
The url here is one done a moment ago on the site you suggested. I can be reasonably certain that if I check it again in, say 2 hours, the speed will be about 65/68 meg.
As an aside, the upload speed varies very little, at any time.

http://schirmfoto.de/d99e021699

As I say, there is not issue about specific times nor have I noticed any lag when I access any site - for example TBB/YouTube/Santander/BG/BT/eBay: all seem fine.
For the record, I live 300 or so yards from my street cab.

The url shows the IP profile - which the BT bod specifically said that if I can see that, then there is no problem on my line and it must be my PC.
Make of that what you will!
Again, any help or suggestions much appreciated

If video is streaming OK then tempted to say it may just be the shared nature of the internet

If it was the PC it would usually be consistent. Know anyone else with Infinity on the same cabinet as you? Is there one nice and stable? If yes take the PC around

Also I would suggest that an 18 inch ethernet cable may put the PC power supply close to the VDSL and be causing errors and retransmits. In theory the ISP can see this sort of data.

In reply to a post by lelboy:

Again, any help or suggestions much appreciated

As you are using ethernet, direct to the HomeHub it might be worth checking if there are any updates for the network driver for the ethernet card, which one is it, and which operating system again?

Also try this speed test:
http://myspeed.visualware.com/index.php

Hi again, Andrew,
Can't tell about any neighbours, as they come and go so often, that I hardly ever know who lives where.
Whenever we use the little laptop - either ethernet or wireless, then the same situation occurs, regarding speed and times. I think that eliminates the desktop.
The performance only started to be extremely variable after I was "upgraded" to 80/20.
Streaming - e.g. 4 iPlayer downloads, concurrent - works well enough, whether I am on a "lower" speed or the 67meg one.
To some degree this is academic inasmuch as there have only been a handful of occasions when the speed was so low as to be unusable.
I only check once or twice a day out of interest - not because the internet is unusable; very, very rarely do I check because poor performance makes me. In truth, most of the time I have no idea what the speed might be, as the net fuctions well enough not to trouble me.
It's mainly that being charged for the 80/20 service - when it's often worse than the 40/10 - rankles both my mind and my pocket!

Cheers, Les.

Shirmfoto url of the test done just now on the BT recommended tester - no other internet activity going at this time downloads etc.

http://schirmfoto.de/f025e02868