IMac freezing with increasing frequency

Question

For the last couple of months my iMac has been crashing with increasing frequency.
The crashes manifest as freezes and seem to random. Sometimes they happen when I'm doing Finder operations, sometimes in Safari, sometimes Mail, sometimes iTunes, sometimes any of a dozen other apps I run regularly. These freezes never seem to leave behind a crash report. Also, after I do a hard boot, sometimes the first one or two attempts to restart also result in a freeze during the startup sequence. Sometimes I only get to the startup gear, sometimes I get to the blue-screen right before the login, sometimes I get as far as half of my stickies popping up and then I freeze again. I've tried Disk Utility -> Repair Disk many, many times (normal SOP for me following a hard boot) and there are never any errors reported. Disk Permissions will occasionally find a file or two that needs to be repaired, but that never seems to make a difference. I've also run the hardware check dozens of times; no errors. I've even fsck'ed a few times to see if that reported any problems that Disk Utility didn't - all to no avail.
I've tried killing all non-essential processes and turning off all of my Login Items, but this didn't help either.
Sometimes I'll restart and everything will be just fine for days, maybe even a week or two, then all of a sudden, freeze after freeze after freeze.
Today has been particularly bad. I haven't been able to keep things going for more than about half an hour. That is, until I restarted in safe mode. In anticipation of possibly having to take my machine into the local Genius Bar (or maybe having to try a clean install), I decided to clean up my desktop and backup a bunch of critical files, etc.. Also, I'm drafting this message in safe mode, since the last two times I tried to do it from a regular boot, Safari froze the machine.
So far, no freezes in safe mode. However, no sound and other restrictions, so this is obviously not a particularly useful mode to operate in...
Before I go any further I should mention that there is another symptom that may or may not be related, but I suspect that it is. Sometimes windows get blocky noise in them. Lots of little rectangles which seem to be copied from elsewhere on the screen. If I force the window to refresh, everything will look OK again, but it might happen again immediately or it might take a while to happen again.
My initial gut reaction was that there was a memory problem in the display RAM, but as I said, the hardware check has not found anything and I tried it again, and again.
So far I have not associated these blocky windows directly with a freeze. In other words, the two events seem to be independent, though they could very well have the same root cause.
Well, that's all I can remember off the top of my head.
Frankly, I getting to be pretty sure that the problem was with the hardware until I booted in safe mode and am not seeing the problem.
I would appreciate any diagnostic advice that anyone would care to offer.

Answer 1

Unfortunately I have had this very experience with freezing/crashing iMacs at TWO different jobs and TWO different computers. In each case the Macs where brand new, out of the box. The first time in 2006, I had started a new job and convinced my boss (a die hard PC fan) that macs where the way to go in the art department. Reluctantly he plunked down for a 24" iMac. Within a couple of days, the crashing and freezing started, randomly at first. Then every few hours-regardless of the application load. Finally, it would run for three or four minutes, than Kaput. Applecare sent a tech who said it was a processor problem. With the processors swapped, the freezing stopped for a few days, then returned with a vengeance. Not only was it a nightmare- seeing as I had sworn up and down to the Admins that Macs where great, but our workflow in the department virtually stopped at this point. Within a week they had me back on a wonky PC. Suffice it to say, my tenure at that company was a short one. The next time about a year later was at a print shop. We had migrated our software and files from our Macbook Pro, Mac mini and g5 (which strangely enough was really slow) to a brand new 24" iMac and within three days, the whole gamut of crashing to complete freeze. It cost the company roughly
$50 000 in lost file processing that week. In both cases we had tried everything from moving the computers to a cooler room, fresh install (Tiger in the first case, Leopard in the second), you name it.
At this point I decided that I should find some work in Graphic Design that just doesn't depend on Macs. I also know a slew of people (industry & personal) with a frightening number of horror stories about their iMacs. All with the same symptoms. This has REALLY shaken my faith in Apple. And even tho I'm a fan and love the OS, I honestly don't think I'd invest in one without a serious overhaul of the line.