Good speed - cannot open web pages

[#1]

Hi,
After weeks of really poor speed  I finally managed to get close to my quoted speed - no help from plusnet here.
Now if I click on "Google" to get the web page it can take a minute or just fail - then click again it just pops up.
Once you are on a page downloads run at full speed - 5Mb for me
Looking at the delays it looks like this happens in 2.5 minute cycles - 2 minutes of slow and about 30 seconds of quick -- is this PC strictly?
Tried the help line - 1 hour wait and you state 5 minute average 
FYI rebooted umpteen times, two routers, new filters, BT's been out (4 engineers, 2 days, cherry pickers, gallons of coffee/tea)
Has anybody got any ideas - I don't expect PN to deliver
Cheers

[#2]

Try on other devices? Also make sure parental control isn't enabled in the router. Could possibly be a DNS issue? 

[#3]

Same problem as me mate, there is no pattern to mine, webpages time out, then they open straight away, then they time out again. I am using Win 8 64 with IE 10. I remember upgrading to IE 10 when I was using Win 7 64 and had the same problem, so I switched back to IE 9. Now though with Win 8, I am stuck with IE 10 as I can't find anyway to upgrade to IE 11.

[#4]

This has the symptoms of intermitant line noise.  When there is no noise, responsiveness is good.  When the nose is present due to errors and retransmission the responsiveness is poor.  When you get poor response, pick up the phone and listen to the phone line.
If yu want to get a definite view of the problem, get and run RiuterStas Lite.  Ths will give you a full picture of SNRM.

[#5]

Thanks all
Tried all of the proposals listed above and those in the assistant
(FYI SNR 6 ± 1 constant for 3 months using Router stats)
It just stopped yesterday about 15:00 no reason except that the apparent upstream events of SNR = 0 have stopped.
The DSL 2740R is a bit old so its a new modem/router
Thanks

[#6]

Router stats lite doesn't work with my router unfortunately Townman, my problem is strange, if I get a web page that won't open, say PN, I can open a new window and carry on surfing with no problem, but the original web page will time out. So I try and open PN in the original window again and it wont open, then I try and open PN in the new window, and it wont open. So while one page is timing out, I can carry on surfing in the other window, but if I try the timing out webpage in the new window too, that will also time out.
If I search for something in the MSN /bing search engine, this will sometimes time out, but if I open a new webpage and search through Google, it will open no problem, and visa versa. So my internet doesn't go slow or become non responsive, it just can affect any webpage, at any time, but I can carry on surfing in a new window, as long as I don't try opening the stuck webpage in the new window.
Hope that makes sense.  
Edit,
Say you are looking at this webpage and have read my reply, then you click on the forums tab to go back to the forums page, it will say waiting for PN until it times out. So you click back and get back to this page again, click on forums tab and again it says waiting for PN, so you click on the home tab, it opens straight away, you click on the polls tab, it opens straight away, you click on the library tab, it opens straight away, you click on the forums tab and it will time out. PN is just an example, it will do it with any webpage at any time.