My iMac G3 Graphite (600) screen goes black. This occurs very randomly but also very often - within 30mins usually.
When this occurs there is a loudish tick/clunk and the white light at the front flickers and you can hear a ticking soung for a while (hardrive maybe).

If the side of the imac is tapped the screen will return but the comp is still frozen.

However (I just found this out) if it is left black for a while - the ticking stops, but the white light still flickers. If tapped at this stage then it wakes up and all appears fine.

The time period may be how long it take to go sleep.

I have updated the firmware.

Any help??

I am not sure specifically but it sounds like your PAV (power-
analog video) board might be going bad. If this is the case
and you have a video port in the back (it will be under a
plastic grill near where the power cable plugs into the
computer) then you can connect an external display into the
iMac and run it just fine there. I'm not sure what else to tell
you though as this might not be the correct solution but it's
all I can think of now.

Good luck!

I have done this already.

When I do this the external monitor does not crash but the
computer still does. i.e. it freezes at the same time as the
monitor goes black but does not affect the video out
signal...

boot up holding down the apple key +option key+O+F
then type reset-nvram press return
then type reset-all press return
trash the caches folder in the user libray and reboot

then go into system perf engery saver and change the settings to never and and uncheck put harddrive to sleep
also boot to ths os disk and run Disk 1st ade
logic board could be going bad.
but i the cracking nosie is cominf from the hard drive it could be a bad harddrive, running DFA will tell you...email me and let me know b101875@aol.com

jel