DNS resolution?

[#1]

Can anyone help with a problem I am continually having with what I think is DNS resolution.  I can often go to browse via either Firefox or IE and find that I just get an error message saying that the server cannot be found. This can happen during a browsing session when all is working okay and then go to the next page or site and I get the error message, and this has been occurring for months and months (on and off). If I reboot my router it normally sorts the problem until the next time which could be weeks, days or hours, but the last few days it has become more frequent and on one occasion (yesterday/Saturday) even a reboot wouldn’t work.  The router normally uses the auto setting from Plusnet for DNS servers, but I ended up having to switch the router config to fixed openDNS servers, disconnect and reconnect and it resumed browsing again. However, just now that config returned the error also and would not work without a reboot and resuming back to the auto Plusnet servers. I am not sure just what is happening but it is now getting very frustrating and I would be grateful of any help please.

[#2]

Try configuring your computer to use plusnet's DNS servers directly, bypassing the dns server in the router. There is a guide here.
What router do you have and how old is it? If it got worse recently, it could be a sign that the router is reaching the end of its life.

[#3]

Thanks very much for that ejs.  I will try the direct DNS and switch it off in the router - and report back.
The router is a DLink DSL-G604T which has been a good router for me up until now. I am trying to understand the logic in why a router would work for the most part but occasionally fail on DNS resolution? 

[#4]

AAMOI The link is for an XP system, do the procedures and details work for Win7?

[#5]

The principle is the same but this link is for Windows 7. (Vista is the same.)

[#6]

Thank you.
In the XP link two specific IP addresses were mentioned. No mention in latest link, so do they apply still?

[#7]

The first was a Plusnet specific link, the second a general Microsoft link.
Try: http://www.plus.net/support/software/dns/changing_dns_windows7.shtml

[#8]

Thanks
Unfortunately it doesn't want to load. Chrome is either telling me 'no data received or hanging.
Tried to load and use an IE add-on, but where it is is a mystery.

[#9]

Do you mean that Plusnet help page is failing to load? The page suggests using preferred DNS settings of 212.159.13.49 and 212.159.13.50 which are both Plusnet IPs. Make the second one 208 . 67 . 220 . 220 if you want to use one Plusnet IP and one OpenDNS IP.
To get to the article does working through the following path help?
First get to the Plusnet Member Centre, then click Help & Support
Click Support Pages, scroll down to the bottom right-hand box and click Windows 7
Scroll down to and click Change DNS Settings to read DNS setting instructions.