I think it's all good now.
Incase other have a similar issue:
Before I updated the bios to 0030, the NUC was left connected to the AC power over night. And it cold-booted fine. This might sound odd, but i feel that when i disconnected the AC, and then cold-boot after reconnecting the AC, the NUC bootup behaves strangely.
In any case - I updated to bios 0030. And over the weekend, tried a cold-boot after leave it disconnected from the AC over night.. and it booted up fine!
Thanks for your help.