Problem with Vista idle detection

I've got a problem with Vista's Idle detection on two machines (one desktop,
one laptop) both running Vista Ultimate.

If either PC is woken from S3 by either a scheduled event (other than a
Media Centre recording), or via WOL, then the machine will always re-enter
S3 mode after 2 minutes.

After spending quite a lot of time Googling around looking for an answer,
I've come across a few other people with the same problem, and also found
the following from a MS document titled "Windows Vista Energy Conservation":

"Improved idle detection that helps ensure that a PC awakened from the
network or for scheduled activity returns to Sleep after 2 minutes of
idleness"

That sounds like my problem, except the PC will always return to sleep
regardless of whether it's idle or not. As a test, I wrote a simple
command line program which keeps the CPU usage at around 50%, ran that, but
the PC still reentered sleep after 2 minutes despite the CPU running at 50%

I've also tried other things such as putting the PC in to high performance
mode as the first thing in the event (using powercfg), but although if I
then check the power mode, it tells me it's in high performance mode, it
still sleeps after 2 minutes.

Can someone from MS acknowledge this please? Is there lightly to be a fix,
or at least a work around? At the moment, it means that my backups aren't
being run reliably as the machine sleeps before they're finished.

Thanks,
Adam.