Monitor Shutdown At Random?

Hi all,

I have the following system

Intel P4 3.06 Ghz HT Enabled
ASUS P4S8X-X (AGP 8X)
512MB PC2700
NVidia 4800 SE AGP 8X
420W PSU
CL Audigy 2
Thermaltake Xaser III Case ( 7 cooling fans! )
Win XP Home

All the latest drivers are installed (including SiS AGP and Mini IDE for
motherboard) but... I have the following problem.

Whilst playing games (UT2003 mostly), The monitor will randomly go into
standby mode (display goes black and monitor power light goes yellow). The
video display then refuses to come back on until the PC is reset and the
monitor turned off and on. It doesn't happen mid game, more usually when
begining a new level / map. I'm guesing it's an AGP or Nvidia issue, but I
can't pin it down.

Any ideas anyone?

One thing I did notice is that the Nvidia 4800SE should support AGP Fast
writes, and there is an option in my Bios for AGP Fast Writes enabled, but
SiSoft Sandra reports AGP Fast Writes are not enabled?

TIA

RJ