Terrible speeds for two days after apparent DNS issues

[#1]

Ok, much as I hate to moan on forums about customer support, I have now contacted support in three separate chat sessions with no resolution.  I've also noticed that no open tickets have been raised on my account as a result of the chat sessions.
So here's the timeline:
Yesterday (31/8) about 10am:  lost all ability to open web pages in a browser.  Skype and Remote Desktop to a colo server not affected.  Seems like DNS issues?
Then at about midday, I can get to websites again.
However, since then, my speeds have been awful.  Since getting FTTC, my speeds have been rock solid at about 70/18, maybe a little less during the evenings when the rest of the street is online, but mostly pretty damn good (with apols to those out in the sticks).
So since midday yesterday, I'm getting an average of about 15/0.5 (yes, zero point five upstream).
Obviously I've rebooted the modem, and the router, and the PC.  And yes, of course I have an ethernet connection.
I spoke to one of you lovely support chappies in a chat session yesterday, and he said that I'd been somehow (?!?!?!?!?!?!?!) reprofiled at 40/2, and that he'd sort it.  However, it ain't sorted, and now I'm getting the runaround being asked to run the standard troubleshooter with its three BT Wholesale tests over yet another day, just to confirm what has already been the case for more than 24 hours - and acknowledged as an issue.

Are there any others who had a mysterious outage yesterday for a couple of hours, that now have link:censored speeds?

Apparently my profile is now looking ok (77 point something, as I was told in my third web chat session with another support bod), but things are far from right.
Would really appreciate some speedy help here - I work from home and my tinterwebz is all but unusable for the things I need.  I'm always telling people how great PlusNet is as an ISP, and how good the service is when something goes wrong - but I need some lurve coming back, ok?
adie:red removed avoidance of swear filter.]

[#2]

Hello
I'm sorry you're not having any joy with the webchat agents.
However, this sounds like something that our faults team should take a look at, this can be raised to them by going to http://faults.plus.net
adie:green removed copy at PN request]

[#3]

Quote

I spoke to one of you lovely support chappies in a chat session yesterday, and he said that I'd been somehow (?!?!?!?!?!?!?!) reprofiled at 40/2, and that he'd sort it.

To put it simply, they made an error. You are definitely profiled correctly, I've copied line test results directly below.
Downstream Speed 79.9 Mbps
Upstream Speed 20.0 Mbps
Profile Name 0.128M-80M Downstream, Retransmission Low - 0.128M-20M Upstream, Error Protection Off
Unfortunately we don't have any results from a BT Speedtest which are needed to report this, can you make sure you've ran the test at http://speedtest.btwholesale.com/ and completed the 'further diagnostics' test after the first completes.

[#4]

Several BT Wholesale tests now run (and added to my open ticket) - this one is typical:
18.84 / 0.7 / 19

[#5]

Did you click the further diagnostics button after you carried them out?
I can't see anything logged on our suppliers systems

[#6]

No, sorry - I have now.

The thing is, as I've explained, this all arose following a very strange outage - no web sites were accessible for two hours, during which time I could still use Skype and RDP (by IP address) to a remote server.
Surely you'd agree that this is too much of a coincidence to be anything to do with an actual line issue?

[#7]

Thanks for carrying that out.
Any chance you can run the next test?
You'll have the option to continue after the first test fails.

[#8]

Please read to the bottom of this post!!!

Ok, so I followed the instructions.... reconfigured the router to connect using speedtest@speedtest_domain.  It appeared to connect ok, but my system tray network icon indicated no internet.
Not surprisingly, all I got when attempting to run Test 3 was 'server not found'.
So I reconnected using my xxxxx@plusdsl.net userid.  Connectivity was restored... and I thought I may as well have a go at Test 3 while connected as myself.
Rather surprisingly, I got this:

So I try a speedtest.net test... and got this:

I repeated it and got 74/18 the second time, so my speeds appear to have recovered.  Immediately after a disconnect/reconnect?  Again, too much of a coincidence.
However, please read on:
I had a bit of a poke around in my adaptor settings, and remembered that I had added Google's DNS server (8.8.8.8 ) to my IP4 config the other day when I got the original problem.  This did not, however, solve the web site accessibility problem and I forgot about it.
Now however, I removed this, and guess what - no web sites were accessible.  Normally I use your DNS servers - 212.159.13.49/50, but these do not currently work for me.  If I use Google's DNS server all now appears to work.
So:  why can't I use your DNS servers?

[#9]

I'm not sure why you can't use the Plusnet DNS servers, but the Test 3 using speedtest@speedtest_domain login details will not work unless you use the automatically assigned DNS servers. Any other DNS servers, like the Internet in general, will not be accessible when logged in using speedtest@speedtest_domain.
The Plusnet DNS servers can only be accessed from Plusnet broadband connections - perhaps check that your public IP address belongs to Plusnet - you can look it up here, in the RIPE Database (Whois) part in the top right corner.
I think the usual Plusnet DNS server IP addresses are 212.159.6.9 and 212.159.6.10, but it should make no difference if you use 212.159.13.49 and .50, all those IP addresses go to the same actual servers anyway.

[#10]

You could try flushing the dns cache that might sort the server issues