Hi,
By the time scan get round to fixing it under warrantee you could end up a few weeks without computer, (if you are sure it's the mobo) get an rma yourself from gigabyte and be up and running in a week,
yes it will cost you p/p of a tenner, but what charges will scan require?
it's looking like a graphics/temp problem,
have you a graphics card that you could install to test?
cheers