Internet connection keeps dropping

I am having a nightmare with my internet connection and would appreciate if anyone could help:

The internet connection seems fine for days at a time, but then it drops for random days. There is other computers including macs on the same network and all of them are unaffected when my Imac loses internet.

When it goes off, I can restore internet by unplugging the ethernet cable and plugging it back in - this restores for about 5-10 seconds and then it goes off again. Sometimes its longer but there doesn't seem to be any pattern to it.

Here are some of the facts:

All other computers are unnafected by the issue - all are connected and working fine (including another mac)
Have tried changing ethernet cables, but this doesnt solve the problem, ruling out ethernet cable as the cause
Opening network preferences it says Ethernet connected & Airport connected - and that airport is connected to our BT businesshub
Have opened disk utility and ran both repair disk permissions and repair disk - but this hasnt changed anything
Have reset the router but no affect
Have updated the software
Have tried bolting on to another guys Iphone network - again it connects for seconds and then doesnt work - so this is independent of the router!
All the network dialogues appear to say that its working - however it just isnt!

This has happened a few months ago - but the problem seemed to fix itself rather than anything I did. It was working fine for a few months but more recently these problems are happening about one a week. Each time, the problem seems to fix iteslf - however I can't find the cause.

Any help or suggestions would be GREATLY appreciated!
Kind regards

Which Mac do you have exactly and which version of OSX? Is it up to date?

Firstly, turn wi-fi off so we aren't trying to deal with two network connections simultaneously.

Next, try logging into a guest account or a new account and see if the issue persists.

Do you have any third party software like a firewall, AV, etc?

Thanks for speedy response.
I turned the airport off after the first instance of the problem, and this made no difference. It is now off.

The Mac is an Imac Intel core i5, 27", 2.8ghz, 1 processor, 4 core - purchased mid 2011.
The version of OSX is 10.6.8 and is up to date.
There is no 3rd party software as far as i am aware.
I have tried logging on as a guest account and the problem is still there.
Have tried restarting everything, however the problem is still there.

Last time it happened it was about a week ago, and the problem lasted for approx 4 hours. During which time, to 'reactivate' my internet access I unplugged the ethernet cable and plugged it back in again. This allowed access for about 20 - 30 seconds, however this was not consistent. Sometimes I would have 10 minutes before the next instance, sometimes an hour, But all the problems were experienced in the same day, then it would go for days without any problem. This morning, its happening again.

Appreciate your concern.

You've ruled out differences in wi-fi and ethernet.
You've ruled out the router as an issue.
You've ruled out a general period of internet connection downtime in that your other machines work
That it's happening for periods of time suggest a piece of software running at intervals.

But to be certain there's nothing at fault with your network configuration can you reset it please:
In Finder go to:
yourMacHD/Library/Preferences/SystemConfiguration/preferences.plist

Rename the preferences.plist to preferences.old

Restart your Mac

Reconnect to your network and try again.

If that doesn't work then do the same thing but for the whole SystemConfiguration folder

Thanks for this. After the exchange yesterday typically it all resolved itself - and as per it seemed to apparently sort itself out. This has happened on a number of occasions now, so next time it happens I will try your recommendation. Thanks again, I will post how I get on.