boblowes

I’m getting exactly the same issue on my recently rebuilt 2009 27” i7 iMac. I had had to replace the graphics card, so installed one from a 2011 model which is compatible, having taken the time to renew the thermal past on the GPU and CPUs. I have read that Sierra and High Sierra have some sort of bug that can cause random restarts - this was introduced in the last version of Sierra (10.12.6) Apparently, it is possible to circumvent this when installing from a USB stick by swapping out to an earlier installer on a separate USB stick, which somehow fools Sierra and installs correctly. Sorry it’s a bit vague - but if you google around for “Sierra random restart bug” and you should be able to find out more.