Start up restarts
QWhen starting or waking my iMac Mid 2011 running 10.11.6 often spontaneously restarts several times before working, even in Safe mode. I’ve reseated and tested memory. There are crash reports referring to GPU resets. Is the graphics card dying? by Kris Lee
AThese restarts are almost certainly caused by repeated kernel panics, most probably reflecting a failing graphics card, but they could result from El Capitan’s fragility.
Run Apple Hardware Test (AHT, or Diagnostics on recent models) first. Shut down and disconnect all external devices apart from wired keyboard and mouse and any wired Ethernet connection. Start up, and immediately press and hold D until the AHT icon appears. Select language, click the right arrow, begin the test. Initially, run normal rather than extended testing. If you can’t get your mouse to work, use the cursor and ® . Sometimes AHT or Diagnostics don’t detect graphics card faults, which should return an error code starting with VFD, typically VFD006. More extensive hardware diagnostics from your local Genius Bar may be needed.
If there’s definitely no hardware fault, browse your logs following starting up. For El Capitan, check out bit.ly/electriclight_readinglogs. Consider upgrading to macOS Sierra, which is more stable in this respect, and better supported.