Paul,
Thanks for the advise regarding Marvel latest drivers; I just rolled
them back to v7.21.1.3 (ASUS Support CD-ROM). I will let you know in
a week or more if this helped. I don't want to jump to conclusions
any more. I've had multimeter hooked on to the power supply for quite
a few hours. Turns out that my Enermax Whisper (350W) provides a
stable output ( +12 == 12.18 Volts, +5 == 5.08 Volts). So I will
stick to it and in them mean time I've installed Ubuntu 5.04 Linux to
see if that works for me. I will work in Linux half the time and in
Windows the other half + I will leave the machine on overnight running
Windows. It froze day before yesterday in the middle of the night but
this was still with the new (problematic) Yukon driver.

Thanks for all good advice again!