Hello, I recently bought a new iMac 5K and am running into a freezing issue. Any help troubleshooting would be greatly appreciated. Here's the info I have: Late 2015 iMac 5K 27" 4 GHz Intel Core i7 24 GB RAM (4 GB x 2 factory, 8 GB x 2 OWC) AMD Radeon R9 M395X 4 GB 512 GB SSD El Capitan 10.11.3 What happens is I'll be using the iMac under normal conditions (browsing web, writing documents, etc. — nothing intensive like gaming) and the screen will freeze completely. Open windows can't be moved, the dock items don't open or animate, even the clock's time freezes. The only thing that still works is I can move the mouse around the screen but can't click on anything, nor can I switch spaces. My only option at that point is to do a hard shutdown by holding the power button. This happens about once every 2 weeks. After restarting, I immediately check the Console's error logs and see two that stand out: Kernel_TODAYS_DATE_COMPUTER_NAME.gpuRestart WindowServer_TODAYS_DATE_COMPUTER_NAME.underspace_watchdog_timeout.spin ("TODAYS_DATE_COMPUTER_NAME" is placeholder for what actually appears there) I'm not educated in reading these type of error logs but in looking at the Kernel file, some of the key info appears to be: Event: GPU Reset Hung Channels: GFX Pending Command from: UnknownCtx GPU Debug Info (which is unreadable) And in the WindowServer file, some highlights are: Command: WindowServer Parent: launchd  Event: service progress watchdog Fan speed: 1202 rpm Then in the Timeline format stacks, I see a lot of reference to the CoreGraphics, CGXServer, and WindowServer Additionally, I noticed that sometimes when I hard restart from these crashes, iTunes shows a "glitchy" black bar over the scrollbar that jumps and shifts as I cycle through music albums. This appears to eventually go away but definitely looks like something wonky going on with the graphics. I did also run the Apple Hardware Diagnostic test and no issues were reported. Based on the above info, is anything standing out as the obvious culprit? And yes, I do have AppleCare, but this is my work computer and to bring it in for repair is quite an extensive process. Ideally, I'd like to first troubleshoot the problem before making decisions about returning or having it looked at by Apple. Thank you for your time!