iMac (2010) Freezing with 8Gb of RAM
Ever since I got my iMac in 2010 (21″ Core i3 3.06Ghz with 8Gb of Ram) it has been randomly freezing. This freeze is permanent, and requires a forced reboot of the machine by holding down of the power button. The screen is stuck on whatever was happening at the time, and it would be completely unresponsive to all input.
I had read several similar stories on the apple forums, all with different solutions. For example, some suggested unplugging all USB devices, others thought it was adobe flash. I tried everything and still I would randomly get crashing. It was the randomness that made it really hard to pinpoint what was going wrong, and also very difficult to get any tech support guys to believe that the machine was faulty. The iMac could go for weeks without problem, and then suddenly you would get 6+ crashes in a day.
I tried stressing the system with a number of benchmarking tools (cinebench, Xbench) to see if I could reproduce the crash/freeze on a regular basis. But this too proved a dead end. That was until I came across Prime95. This is an excellent (FREE) tool for stressing the CPU and Memory of your system. And more importantly it was able to produce a crash with 100% success rate within 15-30min of running the built-in TortureTest.
What was really surprising was that I was only able to get the system to crash when all 8Gb of Memory was installed (see image below for summary). Each individual memory stick seems to be fine, so this suggest that there is a bug in either the OSX software (NB: crashing has occurred on 10.6.4 to .8 and 10.7), or there is a very weird hardware flaw that is only tripped when the maximum RAM is installed.
Software seems the most likely, this post suggest setting the system to 32bit mode solves the problem. When I get some more spare time I will give this ago.
My current solution to this problem is to run with just 6Gb of memory, this has yet to produce a crash after several weeks of use, and several hours of Prime95 stress testing.
In the future I hope that Apple takes notice of this issue and starts looking for a solution.