hi ,
I am using Redhad 8 , I have experence a number of times
that my system suddenly got a screen black up , when
screen show something live a screen saver , but it does not
move and I have to [ctrl-alt-del] to reboot system.

Recently also I experience that when I lock out , not shut down ,system log me out and then it hang , with the keyboard
cap lock and scroll lock light keep on flashing and screen black
out , I have to force reboot...

And I have no idea , when start up I check message seems
no abnormal msg. , any idea ?

I had similar minor screen black-outs with Mandrake 9.0 and RH 8.0 on a notebook. It was a little frustrating on Mandrake. Very minor on RH.

Don't know why it happened though. When I restarted it didnt come back for a while.

RH has been fine for the last couple of weeks.

Try update the latest errata from RHN and see whether the problem still exists?