iMAC crashes/strange polygons appear

Hi,
for about month I have been experiencing multiple crashes on my iMAC. The symptoms:
I start seeing large polygons that usually disappear when I move to another position.
This may happen a couple of times, I call it the black polygon disease although the polygons are not usually black. Some time after that the SL view crashes so hard I have to power cycle the iMAC to reboot. At times the entire screen has gone dead and all I hear is a periodic thumping sound (from the speaker). The one common point is it begins usually on Hobo Island (there are giant prims and other beasts here). The problem can appear after I leave there and go to another sim with the above mentioned results. I wonder if anyone else has run into this kind of problem or does it sound like a hardware problem to you?
Are there any diagnostic in the SL viewer that might help track down the problem?

Thanks
Argo.

BTW I tried clearing the cache without much result.

This sounds like it might be your graphics card warning you it is about to fail. Your shutdown may be the computer preserving itself by shutting down when it hits a high temperature. I would highly recommend that you take it to Apple and have them run diagnostics for you.

I've always had a similar problem with SL, but my graphics card works like a charm with any other application. It seems a pretty clear SL bug to me.

Disable Vertex Buffer Objects in SL's hardware options in the graphics preferences menu.

I'd also give the computer a once over (particularly the VRAM) with Tech Tools or some other diagnostic software.

True but I still recommend a once over at Apple to make sure the processor isn't failing. Hard shutdowns and lockups are never a good sign even if it is happening with only one app. It might be something simple as a fan problem that is causing the heat buildup.

(1) Turn off Vertex Buffer Options in Adv Graphic Preferences, done but it still crashes.
(2) Run diagnostics, I used the diagnostics that came with the iMAC, it got a clean bill of health.
(3) The heat is making the graphics card crazy. Tried keeping the iMAC off until I use it but
the crashes return.
(4) If I stay away from hobo island I do not crash. Go figure, my home is on hobo island.
Just a short visit to hobo island can cause a hard lockup that can happen any time later, even in another sim. Cleared the cache BTW.
This reminds me of what we used to call a "bit spreader," its a software error where some bits are stored accidentally somewhere in memory awaiting the unlucky task that uses that memory location.
(5) This problem is confined to SL (and hobo island so far). I spend hours on the iMAC otherwise without crashing like this. Of course, SL is my most graphic intensive application.
Perhaps there's a debug log I can turn on that will capture what was happening just before the crash?

Thanks
--Argo (perplexed)

I would suggest not wasting your time...
The sad truth is that SL for Mac has always been full of bugs and we can't do anything but wait in vain for LL to rewrite the code from scratch.

at certain places at certain angles, I've seen these polygons too, but its pretty rare... and hasn't ever caused me any problems... have also seen them on multiple machine.

Which iMac and what version of the MacOS and SL viewer are you running?

Is there any particular place on Hobo Island this happens?