Really getting my goat now

[#1]

I'm unable to browse to certain sites.  This has been happening since we took Plusnet and it's getting my goat now.
When it's happened previously, i've changed the MTU of my router and that's done the trick for that particular site but it's not working this time.
It seems to be the sites I visit most that become affected. 
Any clues?  I'm ready for binning this nonsense.

[#2]

could it be a dns issue?

[#3]

If you mention one of the sites you are unable to access others can check to see if it is a common problem.

[#4]

http://community.plus.net/forum/index.php/topic,86642.msg724088.html#msg724088

[#5]

Quote from: spraxyt

If you mention one of the sites you are unable to access others can check to see if it is a common problem.

I am also having similar problems myself today and unable to access some sites.
Example is http://www.personalisedgifts.co.uk
I have checked with the hosting company and my IP address has not been blocked for that site.
I have also tried to access on my laptop and have tried Firefox, IE and Chrome browsers but still no luck.
When I go through a proxy server though the site loads.

[#6]

Quote from: Wardy

I am also having similar problems myself today and unable to access some sites. Example is http://www.personalisedgifts.co.uk

Had no trouble loading and navigating this site

[#7]

I am though, most sites I visit are fine but today there is a handful where I can't get them to load and this is one of the ones which won't load for me.
I have just read this page http://community.plus.net/library/settings/guide-to-rwin-mtu/ and then run the test at http://www.dslreports.com/tweaks and it says this:
"The test server is unreachable
Error: Connection refused: connect"
dick:quote

[#8]

Quote from: Wardy

........When I go through a proxy server though the site loads.

What proxy server?
I reckon about 1 in 100 computers have a rootkit virus ("TDSS") that subverts your normal DNS lookup.  It wouldn't surprise me if these compromised computers showed this sort of problem.