It gets even more puzzling... At least two of the 89% downloads on that screenshot are in the download dir, one fully downloaded and not in need of repair, the other just missing an sfv file that took a split second to restore. When I resume these downloads in SAB, the restored one jumps to 95%, then everything I start (including the restored one) eventually produces the 'out of memory' error again.
I did notice a lot of page faults (1.9M!) in process explorer, even though I restarted SAB a few times this morning:
I've sampled a few other running processes, and they only display a small fraction of these. As mentioned, memtest86 ran a full pass without errors, and so far anything that gets stuck in SAB I can download without any issues with NZBget on the very same machine.
Anything else I can test / try?