Hi there, I am betting it is your PSU. I have heard of this a lot, especially when the G80 based cards first hit the market. People were realizing that their PSU that they 'thought' was sufficient enough really isn't. Despite all voltage readings showing well it does not mean the card likes to be run from that PSU unfortunately. I am betting a newer PSU from Corsair/Silverstone etc will do just fine. It is not the board at all 'I feel', yet the PSU.

Yes I personally leave the CPU FSB jumper on default and it does detect 266 upon post as should. If you are getting something different perhaps flash to a later bios and do a long, hard cmos clear and start again after optimized defaults are loaded in the bios and see what happens.