Slow Internet Explorer access at a specific time of day

Hi All - i wonder if anybody could shed a ray of light please ?

between 08:30 & 10:00 the search facility on Google using IE8 is painfully slow, so slow the predictive results in the search bar take an absolute age to materialise, you could make a cup of coffee and drink it in the time it takes IE to build a page. :-( (using Google Chrome doesn't make any difference)

This only happens on this one PC on my network & is not related to the network - other PCs are perfectly OK.

I have : -

Reformatted hard drive & reinstalled Windows 7 64bit Ultimate + SP1 + M$ updates to date.
Doing a similar fresh installation on anther PC does not reproduce the problem.
Installed windows Security Essentials & have a good result from a scan.
Reloaded the BIOS on the MB & loaded Recommended Defaults.

Any suggestions would be gratefully appreciated.

Many Thanks in advance.

Hi many thanks for your response -the MB is a Gigabyte GA-EP31-DS3L rev 2.0 with a Realtek RTL 8111C LAN controller running the latest drivers 27/09 2011. Interestingly this is definately NIC related, but only between 08:30 & 10:00 GMT - massive amount of network activity, which interestingly has been temporarily cured by installing a cheap independant NIC card.
This makes me wonder, is there a possibility the NIC MAC address is under attack from an external source ?? and if so, is there a possible fix ???