So I loved my old 27" 2010 iMac. Ran great, put in SSD in place of DVD drive, but it was getting old and slower than I wanted so I upgraded. 2015 model, i5, 395 video card, 5k model. Ram to 24gig with OWC. 2tb fusion drive. MY old 24" cinema display worked at first, then did;t. Figured it was old so bought a Dell 2715 4k display. Worked fine, for a day. then it was intermittent, had to do plug shuffle to get it to work (change TB ports for the mini-DP Now it won't work at all. It DOES work in safe mode which makes me think something is up. I started out cleaning out a lot of old KEXT files (Canon, others) and launch daemons, etc. but that hasn't fixed anything. Thoughts? Running 10.11.4 most recent (presumptively GM) Gonna drive me nuts. Next idea is 100gb partition with fresh install of 10.11.3 to see if that fixes it.