Dodgy jumpers on the motherboard for switching between onboard & PCI card graphics. Shuttle sent me some others but by waggling the existing ones I got mine working again. Try the jumpers other way around.
part of email I had with Shuttle:
Dear Shuttle Customer,
Did we change the Jumpers for switching the graphics card from onBoard
to PCI-express in the past? May be this can get faulty after time, if
not changed before.
this sounds very crazy. Do you remember if we replaced those jumpers for
the GPU to new ones? Because from the first series - I think like yours
- have faulty jumpers. Therefore it could be possible that from time to
time they gets a little bit loosen.
Best way you try to move them upside down, to make 100% contact. If this
will help, I'll send you fast new jumpers.
If you still have any questions feel free to contact us.
--
Sincerely Yours
Shuttle Support
Then from me:
Excellent knowledge of your product again
As soon as I tried the jumpers upside down it fires up all the time again with both sets of RAM.
So glad it wasn't anything serious. Can you forward me these new jumpers so I don't get this problem again."