Start up restarts

When I start or wake my iMac (Mid 2011) running 10.11.6, it often spontaneously restarts several times before working, even in Safe mode. I’ve reseated and tested the memory. There are crash reports referring to GPU resets. Is the graphics card dying?

These 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 a 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, and begin the test. Initially, run normal rather than extended testing. If you can’t get your mouse to work, use the cursor and the Return key. Sometimes AHT or Diagnostics won’t detect graphics card faults, which should return an error code starting with VFD, typically VFD006. More extensive diagnostics at a 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.