Hard drive sometimes not recognized! Plz help!

Hello,
For the past 3 weeks I've been having problems with my iMac during start-up, and I'm completely out of ideas to solve them...
Here are the symptoms:
-Very long grey screen at start-up, culminating in the "?" folder after 10 or so minutes.
-When I force the system to start with the CD, 50% of the time Disc Utilty doesn't even see the HD!!!
-When it does see it, I can select it in the Start-up Disk utility and my system then boots up normally.
-When it doesn't see it, I have to restart (using the CD) until it does.
But, in the end, once it starts using the HD, everything works perfectly (until the next restart).
Can someone please help me figure this thing out?
I tried everything I could think of:
-Running Disc Utility from CD: nothing wrong, permissions OK.
-Running the CD in diagnostic mode: nothing wrong.
Thanks a lot you guys!
Loa

If you haven't tried booting in Safe Mode, you could try that.
How to boot in Safe Mode
What is Safe Mode
*"When I forrce the system to start with the CD, 50% of the time Disc Utilty doesn't even see the HD!!"*
You could try reformatting the drive by doing a completely erase and install with your restore disk but there's not guarantee that will work.
If you have another Mac, you could use Firewire Target Disk Mode and try to access your files.
Carolyn

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