P5N-MX - can't see PCI-E card

I recently got this message on my PC when booting up:
>>>>>>>>>>>>>>
Warning System Boot Fail:
Your system last boot fail or POST interupted
Please enter setup to load defaults and reboot again

Press F1 to continue or del to enter setup
>>>>>>>>>>>>>>

I found that if I just pressed F1, it continued to boot and work fine and meant to deal with it sometime, but didn't get round to it...

Then I upgraded my graphics card, and now the board can't see the card - I get the 'no signal' error on my monitor. However, I'm able to use the PC by putting a VGA cable on the on-board socket and I found out:
- entering setup and loading defaults appears to achieve nothing at all
- the PCI-E option for priority of card is ticked in the bios settings (others are: PCI and onboard graphics)
- I've flashed the bios to the latest version - no change
- I've changed the CMOS battery - no change
- I've checked the card on another PC - it's fine (and I didn't find any obvious compatibility problems on the web for the Radeon HD 6870 and the Asus P5N-MX motherboard)
- Device manager can't see the card
- a hardware scan doesn't see the card
- on one occasion of booting and re-booting I managed to get the card to work - yippee! - but as soon as I re-booted it went back to not seeing the card again - and I have no idea of what I did to get it to work

I'm reluctant to disable the on-board graphics, because then I wont be able to see anything, so I'll be totally screwed.

I'm now out of ideas.... anyone help?

UPDATE:
I went through various testing routines for all the components again and discovered that if I add a VGA convertor to my DVI lead it will 'see' and

work with a VGA screen, plus Device Manager now recognises the card is there. THEN, if I take the VGA connector off and connect to a DVI socket on

the same screen it all works perfectly. Bizarre. Anyone know why this would work???