Hi Paul,
Next time it happens, open a cmd prompt and enter "tracert".
Type address of offending site, it should count the router hops to the failure point. Some internet routers will not display stats, * will appear.
Also, in XP, there is an option to "repair" the network connection.
This will flush the cache and reload your address.
Does your DNS point to the router? or to the ISP?
Also, never hurts to have a secondary DNS pointing to a different ISP, incase your ISP is having problems.
