XP Pro SP2 Power Management Issues

I have a set of 3 Dell Precision 390 workstations I'm having a time
with. I have disabled any "low power" settings I am aware of, set the
BIOS to turn off the power conservation settings (to enable "more
functionality"), set the power settings in XP to "Always on", and
configured all sub-settings to "Never" (including System Standby),
disabled Hibernation, etc. Basically, I've removed any possibility
(that I'm aware of) that the system would go into a low-power state.

Well, these 3 machines are STILL doing that. Here's my quandry: they
have to be quick-response machines because they are used by physicians
in our hospital for patient care. Currently, the PCs take several
minutes to "wake up" from the low-power state; occasionally, they
won't "wake up" at all, & have to be manually powered down & back on.
At first I thought it was a hardware issue, but when I discovered it
was happening to all 3, I knew something was up. They are stand-alone
machines, which means the EZ-GPO power management options that I use
w/ all our other PCs via AD don't apply. So, my question to everyone
is this - am I missing something? Is there some power management
setting in the BIOS or in XP that I'm missing somewhere, or some other
utility/tweak/tool (preferably free) that I can use to FORCE those PCs
to stay on at all times?

PC: Dell Precision Workstation 390
Monitor: Dell 24" widescreen, using DVI input

Any help would be greatly appreciated.