Hi @ChrisH
#13 [b]...So I concluded it is definitely a temperature issue (although it might still be a slightly faulty component triggered by high temp, but proving that would be hard).[]
FWIW, from this A1-XE-G4, My UBoot won't boot at below 73'F without at least three minutes warmup.
I have a CoolerMaster case & gauges and calibrated temp probe in the CPU base.
http://www.flickr.com/photos/11367727@N07/2716912145/ As for too hot limit, 45'C is my limit cutout based on Quake2 load. Always choked and sputtered playing Quake2 because on the puny cooling system. Added a Zelman fan and ended that. I haven't foggiest what's required for SAM. You might want to use a micro fan; I would.