hey mate
definitely sounds like a GFX problem... Is it an onboard one? Do you know anyone who has a GFX card u could borrow to confirm the diagnosis?
I wouldnt go replacing the mobo without testing the GFX card first.
I was going to suggest a system restore but looks like u have tried that already... maybe see if u can get updated drivers for the existing one aswell?
give the above a shot and let us know how you go
good luck