I post the following to help other Mac users learn from the problems I had and finally resolved after much trial and tribulation. Let me start at the beginning... Along time ago, about six years ago, I purchased a late 2009 iMac 27", 1Tb drive, and I immediately upgraded the RAM to a full 16Gb (4x4Gb sticks). It ran flawlessly for many years. Then, about a year ago, my trusty iMac began to develop erratic and random crashes once every few weeks. The crashes persisted and increased, and began corrupting my Outlook email. I lived with the problem, rebuilding the Outlook database, and thinking it must be crappy software compatibility issues on an older computer. As the frequency of crashes increased to several times a week, my problems got worse when my TimeMachine backups became unusable. I then figured my iMac had a hardware issue, probably a bad hard drive or maybe a bad power supply. In fact I ended up returning 2 external hard drives to Western Digital under warrantee since Disk Utilities could not fix them and they were making strange clicking noises. I tried reinstalling the operating system (multiple times), took the computer to the Genius Bar twice, replaced the internal hard drive (twice), replaced the optical drive with an SSD drive (amazing upgrade!!!) and moved the OS to the SSD drive, even hired a really experienced Mac consultant. Nothing I tried worked. My crashes continued and increased with frequency to daily crashes to multiple times a day crashes. This iMac was literally wasting my life away as I tried every thing I could think of to resolve it. The crashes got so bad and frequent, I could not get anything done, even backing up the computer. Finally, after having SEVEN crashes in one day, I decided to call it quits and reluctantly ordered a new iMac Retina. As I waited for my iMac Retina to get built and delivered, and still not quite completely defeated by my once trusty iMac, I did a few web searches for random crashes on an iMac to see if others had the problems I was having. Everything I had found were issues I had addressed or didn't think concerned me. One forum I looked at (it was here in Mac Rumors but I do not recall which) said bad RAM could cause random crashes, and mentioned to run RAM check software on new RAM. I initially discounted this a advise, thinking my RAM was 6 years old and had been running perfectly for YEARS. But at my whits end, I downloaded and ran the app Rember just to see if this might be a problem. I was so certain this was just another waste of time down another dead end. Sure enough, within minutes, the tests showed faulty RAM. I was ecstatic: I finally found the problem!!! I decided I would find the exact problem RAM stick that was causing me such frustration. After about half a dozen boot ups switching out my RAM in different positions and running ever more RAM tests, I isolated the problem to one RAM stick. Once I removed the faulty RAM from the computer, this completely fixed all my crashes. It's been over 1 week with my computer running 24/7 and not one single crash!!! I've even tried running tons of apps all at once. No crashes!!! Initially I was running just 2 of the 4 RAM sticks. Some further research suggested I could run just fine with 3 identical RAM sticks, so I tried this. I'm pleased to report that my iMac is purring along with 12Gb of RAM on 3x4GB sticks. And still, no crashes! I hope this story of woes and finally success helps other Mac uses who are pulling their hair out with random crashes. It might just be bad RAM.