My A4000d motherboard started flaking on me so I sent it to Anthony for repair, Anthony repaired it and before installing back in the case I changed the cpu to an A3640...
I am happy to report that I no longer have the problem with the Deneb!
I know i've muddied the waters by having the motherboard repaired, but I'm willing to bet Alexh was right and it was all down to the cpu board...
The repairs on the motherboard were to the rtc, ram tracks and caps...