Computer Forums - Imedia 1529 won't POST

Hi All!
I have a very strange problem with an imedia 1529.
It was brought to me because it wasn't getting beyond fans starting, disks coming up. There was no 'power on' beep.
I figured it was the motherboard and probably the graphics card fried, and sure enough when I fitted new ones and cleared out the dust which caused the problem it fired up properly. I ran it for a couple of days and called the owner to collect it.
After a couple of days' use it seized up again, and after some cleaning of contacts and reseating cpu, memory etc, and putting arctic silver on the cpu it restarted.
I have now got it working three times, each time for several hours, and each time when it was returned to the owner it refused to start, with the same symptoms. I have tried running the board outside the case, with no extension cards, to see if it was a shorting problem (it wasn't), removing the cpu, memory to see if I get beeps (I don't), and trying another PSU, resetting the bios and renewing the battery, all with the same results.
Am I missing something Really Obvious?
Many thanks for any input on this.
PS I have very little hair left...!