I'm running a Radeon R9 390 with three monitors in portrait on driver version 17.11.1.
I have an Oculus Rift and in the last 3 months or so they released an update to their software which makes it so I can't use my Rift without updating to the latest AMD drivers, so I went and installed 18.2.1. Annoyingly though, as soon as the installation was complete my computer restarted and gets caught in an infinite restart loop. I had to unplug one of my three monitors, which disabled Eyefinity and the restarting issue stopped. As soon as I plugged the monitor back in though, the restarting happened again. I had to unplug a monitor, uninstall the drivers and go back to 17.11.1. After that point, no more issues.
This is happening on both my Windows 7 and Windows 10 installations and I've tried various driver versions in the 18 series, all with the same results.
Full hardware specs according to CPU-Z: Intel Core i7 5820K @ 3397.99 MHz - CPU-Z VALIDATOR