I'm having freezing or lock-up issues with my iMac (Retina 5K, 27-inch, Late 2015). Its specs are: processor: 4 GHz Intel Core i7; memory: 32 GB 1867 MHz DDR3; graphics: AMD Radeon R9 M395X 4096 MB; storage: 1TB SSD (Apple) currently with 459 GB available; macOS installed: "High Sierra" 10.13.5 (17F77). The mouse is an Apple Bluetooth magic mouse. The keyboard is an extended Apple keyboard connected by a USB cable. What happens is that, without warning, the mouse and keyboard become frozen. I move the mouse but the screen cursor doesn't move. Then, after maybe 30 seconds or so (it varies), the cursor will suddenly be responsive again, but only to stop again seconds later. The keyboard is similarly unresponsive. When the freezes occur, the seconds on the menubar clock sometimes keep ticking, sometimes not, though it's hard to tell if that's during a freeze or not. If I'm playing audio, that will keep playing during a freeze. I have to wait for one of the "windows" of responsiveness to try to quit running apps, though this isn't always possible because they are so short. Often I'm just forced to shut down the iMac with the power button and restart. I never know when it's going to happen, though I get the impression (maybe not accurate) that it's more likely to happen after the iMac's been on for a few hours, rather than happening soon after a restart. It doesn't seem to happen when I'm doing any specific task or running any specific application. It's a significant problem because obviously I can lose work. Also, being random, it's kind of puts you on edge, wondering whether it's going to happen. I've reset the NVRAM and the SMC, but the problem continues. I'm keen to know if anyone else has experienced this problem, and whether it's likely to be a software or a hardware related issue.