M17x R3 More graphics Card Issues

My trusty M17X R3 has developed an issue with the Radeon HD6990M; it won't boot! The machine works fine with the card removed and set on the integrated graphics, but as soon as I put the card in the fun and games begin.

The problem started with issues experienced on booting up from off, including a blue screen with light blue vertical lines, an orange screen with yellow lines, no video at all on on one occasion 6 beeps. Initial diagnosis suggested it was the 6990M, so I took it out and forced a start on the IGP (I have the A12 unlocked BIOS) and all was well.

On re-installing the 6990M the machine will not even post. The power light and LEDs come on, the screen does not switch on and there is a faint tapping sound from the speaker and that's about it. With the bottom off I can see the graphics card fan spin briefly then stop; the CPU fan spins accelerating then decelerating in line with popping sound. Remove the 6990M and all is well.

Thinking that it must be a grfx card at fault I purchased a replacement 6990M which was advertised as known working. I received it today, installed it and I get the same symptoms; LEDs, black screen and popping with no POST. I've popped the CMOS battery and held the power button for 30 seconds, no change.

Back now to the integrated grfx to type this post. Anyone got any ideas?