Well (to paraphrase myself) that worked, TWICE!
Yup, it was the laptop closing the port. I don't blame the Eee PC1000H itself, it probably just needs a fresh install of the OS. Dunno why, but I'm using a different computer now (also XP SP3) and everything's running like butter wouldn't melt.
Sincere thanks for all the suggestions which have helped to stabilise my electronics and tighten my coding.
Thank you!
[RESOLVED] Near-enough 