Extremely slow 20" iMac G5 (iSight model) Help needed!!

Question

Hi Everyone
Okay, I've spent the best part of the last two days troubleshoooting this machine for a friend of mine.
SYMPTOM
Her iMac is running slowly fullstop. It boots slowly and trying to perform any functions only results in the "beachball" pointer for a long time before the computer responds. Example: Clicking on the Apple Menu can take over a minute to display the options. Select an option and you wait over another minute before the computer displays the result. This is the same for anything you do - within the Finder or any other app!
WHAT I'VE DONE SO FAR
Okay, here is a list of everything that I've done so far:
Disk Utility: Repaired disk permissions 3 times whilst booted in safe mode. No improvement!
Disk Utility: Repaired disk permissions 2 times whilst booted in normal mode. No improvement!
Disk Utility: Repaired disk 2 times whilst booted in normal mode. No improvement!
Zapped PRAM 3 times. No improvement!
Decided to take the plunge and eliminate the HD and OS as possible culprits:
Started computer in Target Disk Mode and connected it to a PowerBook G4 (i.e. another PPC Mac)
performed a backup
Disk Utility: reformatted hard drive 3 times
TechTool Pro: Read/Write Test on hard drive - passed!
TechTool Pro: Surface Scan on entire hard drive - passed! (yes, all 488,397,168 blocks after 17hrs 25mins)
reinstalled Mac OS X Leopard (10.5.6) from an original Apple retail DVD
Booted iMac - now starts up in about 3mins 30 secs (as opposed to a lot longer earlier)Tried using the Finder to perform basic Mac OS X functions (e.g. open system preferences, access About This Mac, etc). STILL TAKES A LONG TIME - Very little improvement!!!
Removed the 2GB DDR2 SDRAM from iMac's RAM slot
Booted iMac - still starts up in about 3mins 30 secs
Tried using the Finder to perform basic Mac OS X functions (e.g. open system preferences, access About This Mac, etc). STILL TAKES A LONG TIME - no improvement over having the extra RAM installed!!!
Re-inserted the extra 2GB RAM DIMM back into iMac
Ran TechTool Pro to test hardware (everything passes, including memory).
Okay, so now I'm at a loss. Any suggestions would be most welcome and appreciated!
Thanks!
Joe.

Answer 1

Actually, I haven't tried Rember, thinking that since the computer displayed the same behavior without the extra SDRAM installed as it does with it installed, that it couldn't be the RAM. And the factory RAM is soldered on, so I can't do anything about that. Still, RAM is what I was suspecting originally (after the HD), so I'll go ahead now and run Rember on it.
Since you're online, I'd appreciate your opinion on something else I've identified. Since reinstalling the system, the computer boots a lot quicker than it was, but as I said it is still extremely slow performing functions once booted. HOWEVER, I've noticed that now once a particular function is performed once, it can be performed again and again quickly.
EXAMPLE
Clicking on the Finder icon in the Dock the first time takes the computer 26 seconds to open a finder window. Do it again and it appears instantly. Likewise, selecting clicking on the Trash icon in the Dock took 31 seconds the first time to open it up. After that it opens up instantly. It's the same with using the Apple Menu, etc.
If I reboot the computer, the same thing happens all over again. Any ideas what this may mean?
Thanks very much for your help!
Joe.