iMac not waking up from sleep mode (10.5.6?)

I believe I am having the same problems described here - my 24inch Imac won't wake up from sleep (stays as a black screen). I solve the problem each time by powering down and then doing a PRAM restart. I changed the RAM and this stopped the problem for about two months which is interesting, but then it restarted again. A solution would be really appreciated!!

Update to previous post: I have tried the following fix and it have had no recurrence of the problem for several days now. Removed the power cord from the mac, waited 30 minutes, then reset the PRAM, holding all four keys until three chimes are heard.

I'm having this same problem, albeit under 10.5.8 and with a 24" iMac G5.

The problem almost always happens overnight. Computer won't awaken from sleep, when I hard restart it seems to indicate an OSX error had occurred. It doesn't happen every night, but more like every 3-5 days. When the system sleeps during the day, no problem awakening it.

I called AppleCare about this, they had me reset the PRAM and do a hardware test. All tests were fine. Repairing permissions does not seem to help. The higher level specialist I spoke to gave me his phone/extension and had asked me to call him directly after the hardware test, and I have done so 4 or 5 times, leaving a message that never garners a response.

As an aside, the "Warning: SUID file " ________" has been modified and will not be repaired" errors have always appeared with Disk Utility under 10.5.X, and continue to despite any fix that updates were supposed to fix. They may be harmless, but I find it coincidental that I'm having this problem and those errors remain.

Went to the Apple store near me, floor rep suggested I wipe the disk completely and reinstall the system (not archive & reinstall), then restore from Time Machine. A bit drastic for my tastes, especially at a time when I'm particularly busy working on the machine and can't afford all the inevitable bumps along that road.

In addition, doing a disk utility scan on my external Firewire drive - LaCie 2 TB Big Disk Extreme+, FW 800, used only for Time Machine backups - had shown a corruption there. I erased that disk and now have it connected only by USB2. This problem of not waking from sleep/system crashes continues.

FYI, disk is 1 TB and has about 100GB empty space. 3.06 GHz Intel Core 2 Duo. System has 4GB DRAM.

Any thoughts would be most appreciated.

Much thanks.