Anyone having issues with external monitors since upgrading to Sierra? I am running a 5K iMac and have had a Dell 4k monitor running off it over DP. Had no issues till now. Since going to Sierra, have had several episodes of the Dell losing the correct orientation and crashing at higher res settings. Worse, it also has a habit of crashing the iMac such that both screens go blank and are unrecoverable without a (hard) reboot... Oh and a lot more fan noise since Sierra. None of these issue on El Cap... Hmph.