iMac keeps randomly restarting

"I think you started down the right path as being a power issue. Given the fact the Apple tech couldn't get it to fail when he had it I suspect your issue is within your homes power not your system.

First you'll need to get a cheap AC Outlet Tester to check your outlet to see if the the outlet is correctly wired. You could also have a grounding issue if you don't have a three prong outlet. This is were you will need to call in your local electrician to fix the outlet. Even still you do need to make sure the outlets wiring is correct (don't cheat here, you need all three wires). Following the wiring back to your fuse/breaker panel make sure you have a 20Amp circuit for 120volts and ideally you should have a dedicated line for your computer and your peripheral gear. Make sure the buildings Ground circuit is in good shape. Often a grounding rod stuck into the ground or strapped to the metal water line feeding the building. Over time the wire and/or the connection point corrode (green stuff) and need to be cleaned. The rod can also have degraded and may need to be replaced or positioned in a better location to be effective. Many people put electricians grease on the exposed surfaces of the connections after wiring it up to help prevent corrosion.

So we went as far as the fuse/breaker panel and we still have problems. You may need to get a UPS to stabilize the power as your power provider maybe having problems giving you reliable power. Many of the better units have power meters which can show you how good/bad your power is.

You could ask your utility provider to test your power:

Have them put a power monitor in your house for a good week or so. They may need to service your homes power feed or replace a transformer that feeds your neighborhood."

My iMac (27-inch, Mid 2011) had the same rebooting problem. It would reboot several times after Sleeping or if powered on. However, after a few self initiated reboots the iMac would be stable as long as I kept it on. I tried installing a different brand (WD) new SSD and worked upwards from Yosemite to High Sierra. I also swapped various certified sodimm memories and I even replaced the logic board. All with no positive effect - the rebooting fault persisted. Further, video testing and memory testing apps always proved the hardware was good. Finally, I put my original logic board back in and this time I physically removed the WiFi card and InfraRed Card. Previously, I has disabled the WiFi via the Operating System without result. With the WiFi and infrared cards removed, my system is now stable. I suspect the fault is with the WiFi card as I recall having a similar problem with the wifi card in my old 17” 2006 model macbook pro. I had to remove its wifi card to solve it. Currently using the ethernet LAN cable.

"On the iMac (21.5-inch, Late 2009) I noticed that the built in wireless would go out after a few minutes of operation and thereafter would reboot. Having tried all other suggestions here and other sites and opening up iMac to clean and check boards I found that turning off the Apple wifi resolved the repetitive rebooting. I installed a wifi adapter card and solved this iMac’s problem.

Also found the Apple fan control very useful to keep temperatures down.

Hope this helps."

"I have the same problem with a refurbed model I bought from BestBuy.ca with 4 years of Bestbuy warranty... The computer clicks and shuts off as if the cord got unplugged, then reboots. Sometimes happens during the boot process, other times during regular usage (though seems to be frequently triggered by GPU usage... but not consistent).

Same results with Lion, Mavericks and Yosemite. Pretty sure it's hardware.

BestBuy Geeksquad replaced the logic board, and it still crashes. I, like you, have tried different cord, different outlet, with and without UPS, all with the same random results. Other 2008 iMac in same UPS is solid. I have to assume it's the power supply.

Going back to best buy tomorrow for them to send away again. Their no-lemon policy states that if they replace hardware 3 times, they'll give me a whole new equivalent or new current model as a replacement. I'm looking forward to my brand new 27"" iMac soon!!"