10.6.3 - Problems and Attempts at Resolution - Please Help

Question

It was recommended to me by a regular contributor (RC-R) to put together in one place all the problems I've had and all the solutions I've tried. So here goes.
Problems:
(1) Major issue is that my 20" Alu iMac is randomly sent into a sleep mode from which it cannot be awoken - it happens instantly and without warning.
(2) The issue started immediately after installing 10.6.3 and nothing like it occurred beforehand.
(3) The issue seems acerbated by using Safari and often occurs within a few minutes of using Apple's web browser.
(4) Long stretches of avoiding the problem seem possible if using Firefox or Chrome instead.
(5) Using switching and start-up take longer and sometimes freeze altogether - everything is slower.
Attempts at restoring normality:
(1) First of all I disable and then finally uninstalled Rapport banking security software - since this was one of the last programs to be installed and I'd seen reports of problems elsewhere - thought this might be why Safari brought the system down. Didn't resolve the problem
(2) I'd been having problems with Adobe's installer unable to upgrade Acrobat, and in order to resolve this I reinstalled entirely and got everything cleaned up so there was then no upgrade problems. This didn't resolve the problem either
(3) I did a reinstall of Snow Leopard and installed all updates again. Still no resolution
(4) I've also repaired permissions on the disk. But again, you guessed it - still crashes
Note: I have a MacBook (Alu) with 10.6.3 installed - no problems whatsoever and the programs installed are virtually identical with the exception of VMWare Fusion.
My Apple Care is close to an end for the machine, so my next step seems to be get the iMac ready to return to Apple before it is too late. Because it looks like a software issue but is acting like a hardware one. The reason I'm assuming it is not a coincidence is because others have had similar problems

Answer 1

Tried to reinstall 10.6 from DVD, but installation failed - twice.
Failed how?
What now? Disk Utility? Format Hard-drive? What is the best way of going about these things?
It's hard to give any specific recommendations without more information. You could simply have a third-party app that's acting up, or maybe your past reinstalls weren't done correctly. So it's possible that there are less drastic solutions than what I'm about to tell you to do. But, as you say, you just need it working, and sometimes a good "nuke & pave" is quicker than fiddling around.
Make sure you've got a good backup (see my [Mac Backup Guide|http://www.reedcorner.net/thomas/guides/backups>). Then, boot from the install disk and switch to Disk Utility. Erase the hard drive. When done, just for the heck of it, repair the drive. If there are problems, post back here.
Now quit Disk Utility and reinstall the system. Once you're done and booted into the new system, run Software Update and install all updates. After that's done and you've rebooted, do it again, continuing until Software Update says you're up-to-date. Now install any third-party apps from scratch, avoiding any initially that you don't absolutely need. (Install those later, once you know your system is stable, and has been for a while.)
Finally, import your data into the new account. I recommend the manual route, though you could also use Migration Assistant to import from a Time Machine backup or clone. If you use Migration assistant, you have to be cautious not to import a bunch of potentially damaged system settings files. (You'll want to reconfigure your network settings from scratch, for example, rather than importing those settings from the backup.) If you want to go the manual route but aren't sure where to find everything, let us know.