I have an MacBook Pro 15 inch Mid-2009. It was bought used on ebay a long time ago, so don't ask about that, because I didn't buy it and I don't remember. Ever since I can remember, this computer was a crashaholic. It would (and still does) randomly crash. Almost always the crash report would contain: Kernel extensions in backtrace: AppleIntelCPUPowerManagement.... bla bla bla It didn't bother me because I installed this FanControl software, and set all three sliders all the way to the left. With this software, my computer rarely crashed (it still did though but not enough to bother me). I recently upgraded to Yosemite, and Yosemite changed some things with the fan stuff and broke all fan control softwares. I removed FanControl. Now my computer started crashing like it used to. I found that it crashes much more often when the fans are at low speed. Using smcFanControl (which is still not updated and broken, but still kinda works) I set the fan speeds to ~4,000 rpm. It still seems to crash more often than when i had FanControl. A while ago I tried installing and running Ubuntu Linux to see if that would crash, but Ubuntu didn't work on my machine. Graphics wouldn't render properly and I couldn't see anything. Today I discovered I could install Mac OS X on my external drive. I downloaded Mountain Lion, and restored the Install thing disk image to a USB stick. It didn't work though, the apple logo turned into a NO-sign (circle with line in it). Then I remembered I had this Lion Recovery/Install USB stick. I tried that and it worked, and successfully installed it on my external drive. I booted from Lion and it worked! I am typing this using Lion on my external drive right now. And the weird this is, I have not gotten a single crash! So, How do I fix this crashing problem? I thought, I really thought it was a hardware problem, but it was not! Do I have to reinstall OS X? Should I downgrade to Lion? (I kinda like Lion better than Mavericks or Yosemite...) By the way, this crashing problem, I believe, has existed ever since Lion was on this. I am not sure if it crashed before Lion, I do not remember. Another thing, when we got this computer, it had an operating system already installed and already used by the previous owner. It was not reinstalled (and yes, I know, it should've been reinstalled :/). If you want a complete crash report tell me and I will boot in the Yosemite and wait for a crash.