Cannot log into Messages or Facetime

Hi all

I am going to give this one last shot before I give up forever.

Around July last year, both Facetime and Messages stopped working on my 2010 iMac. I cannot think of a specific event that may have caused this (change of router, ISP, settings etc).

When I try to log into them, i get a "cannot connect, check your network connection" error. I have scoured these pages and have seen that there are many people that have had this issue and there are a number of possible solutions (automatic time and date, missing serial number, DNS settings etc) and i have tried them but am still unable to log in.

I have gone so far as to do a clean install of OSX but was still unable to log in and got the same message. I have tried using a different route, ISP, connecting the iMac to the router rather than wifi but nothing is working.

Both work fine on my iPhone and iPad (fortunately) so I am now at a complete loss as to what could be the problem. Has anyone else got to this stage and figured out what the issue was?

The only thing left is that when my iMac was a month old I discovered that the Firewire port was not working and the motherboard was replaced. When i check "about my mac" it shows a serial number but was wondering if this is the serial number of the new or the original motherboard or whether i am barking up the wrong tree.

Please help!

Thanks

Paul