Is it maybe not because BT are like, moments away from your telephone line with a pair of wire cutters 24 hours/day?
Well, that would do it.
'Windows could not finish repairing the problem because the following action cannot be completed:
Clearing DNS Cache.'
The fact that your web browsing works for a short time indicates that your DNS settings are correct. Something else is getting in the way. It could theoretically be the cache ... but I've never come across this actually being the reason. Normally if your cache is fubar'ed, your web browsing won't work at all.
You can still try and clear it manually if you like.
Open a command prompt, and at the prompt, type
ipconfig /flushdnsBut I'd say the problem is most likely to be one of three things:-
1. Your ISP being (probably temporarily) crap. If this is the case, you can do little about it.
2. Spyware/Adware. TCPView is great for spotting programs which are abusing your internets. Or you could go for the Spybot & Ad-aware combi.
3. A problem with your registry, most likely caused by Spyware/Adware which may even no longer be present. I've seen a few 'cleaned' machines (usually student's laptops) with problems of this nature. Have you removed any nasties lately?
And lastly, although I always thought of Zonealarm as a pretty good (free) firewall, I recently got rid of the free edition from my home PC because it started causing lag issues similar to your own --- although my symptoms were not quite as bad as the ones you describe.
Oh and I just thought, it could be a hardware problem with your router or modem too. But it's not very likely. Usually when they break, they break totally.