Grey screen w/ white bars; also, random shutdowns

[steeeeve]

I have a late 2012 iMac 27" running 10.13.6, with a 2.9 GHz Intel Core i5. When it sleeps for more than 3-4 hours, on awakening, there's a grey screen with a progression of white bars across the bottom, then the enter password screen, which takes a while for the password bar to highlight. This whole process is VERY slow. What causes the grey screen w/ white bars, and how can I coax it into working better?

Also, lately it's begun to shut off abruptly unpredictably. Sometimes it will for for hours, sometimes it'll only be half an hour or so and right in the middle of doing something, the screen goes black and the machine has shut down completely. I have to switch off the surge protector and wait a good while before it's willing ro start back up.

Any ideas? Thanks so much!!

 

[Cory Cooper]

Hello and welcome.

That sounds similar to this post in Apple Discussions: iMac screen gray after sleep with bars on bottom, mouse and keyboard disconnect. Try the steps in the response there and see if that fixes it.

The shutdown issue could be a thermal issue, if the vents are blocked by dust/debris, or if the hardware is starting to fail.

C