What about your powersaving and screen blanking settings in your BIOS? Are they set to take no action in every situation?
There aren't any such settings in the BIOS at all, oldschool - as I said, this is an older computer. It dates from 1999, and I think that in itself is the real problem, for it has been my experience that PCLinuxOS does not get along with 'old' computers very well at all. That is also likely the reason that no-one else is reporting similar, as very few people now would be running such 'old' gear anyway.
Anyhow; I think I may have resolved it now. Your mention of hardware was the key, as I recall similar issues with 'early' ACPI versions in the past. My guess is that PowerDevil is written to use only the latest versions of ACPI, and that non-PowerDevil power management would likely fall back to using APM instead - so; by disabling everything related to PowerDevil and using _only_ the basic display power management settings, at last it finally seems to be working? At least it hasn't blanked out on me in a while... so; I'm going to leave it going all evening, and if this Firefox instance is still there to greet me in the morning, unblanked, _then_ I may finally be able to mark this problem as [SOLVED].