Late 2012 iMac acting up

[JD2]

Hi all

I am directing myself to the late 2012 iMac 27" owners.
Mine is an i7 version with 8+16G memory installed (8 original,16 OWC added)
I had zero problems with my faithful companion for so many years but the last month or so it has been acting up.
Out of the blue I got the dreaded white screen indicating a problem in 4 or so languages asking me to press any key.
Upon doing that the iMac relaunches from the Apple logo without chime.
I have tried to let it run with almost nothing connected (I have my doubts about a Seagate desktop expansion USB 3.0 drive that sometimes gives me countless messages as if it had been disconnected the wrong way)
But even without that drive the problem keeps occuring.
At a certain moment I thought I had found the culprit because I changed some ethernet connections lately in the way that the iMac was first connected directly to my VDSL-2 vectoring modem and later it was connected to one of the ports on an Time Capsule, but the initial impression was wrong, connecting the iMac back as is was does not solve the problem.
I get the chrash reports that I can send to Apple but I am not an expert on those so the leave me a bit in the dark, on some occasions there is something in there as below but surely not always:

*** Panic Report ***

panic(cpu 4 caller 0xffffff800236ea9a): "Possible memory corruption: pmap_pv_remove(0xffffff803c189500, 0x7fffaee64000, 0x6361fc, 0x80000006361fc024, 0xffffff9ae8cf3c04, 0xfffffe93a297b320): pv not on hash, head: 0xffffff80318a5390, 0x7fffaee64000, priors: 2"@/BuildRoot/Library/Caches/com.apple.xbs/Sources/xnu/xnu-4570.20.62/osfmk/i386/pmap_internal.h:859

Backtrace (CPU 4), Frame : Return Address​

I've read about failing power supplies and motherboards but it is guesswork.
What happens mostly is I hear a faint 'thick' I suspect from the speakers and the screen goes full black.
A moment later the white warning screen comes up.

When I have had the machine off for a day or so this happens basically within a minute after logging in.
Sometimes 2, 3 times in a row, thereafter the machine runs normally (I am typing on it) when I leave it un it will mostly run for a night and sometimes have a crash in the morning but recovering from that afterwards.

Could this be a drive problem? I have an original 1TB Fusion drive (128MB Apple + 1TB WD)
I am thinking about trying to let it run on a bootable USB3.0 backup but I am not sure if this will fully mask a problem with the internal drives.
I am asking about the drives because a was already thinking about opening the machine and update the drive(s)

Anyone any suggestions, they will be greatly appreciated!

JD
[doublepost=1511993910][/doublepost]I forgot, I am running the latest Mac OS version 10.13.1.

 

[Fishrrman]

Have you tried removing the OWC RAM, and just running with the factory-original RAM a day or two?

If not, I strongly suggest that you do this...

 

[JD2]

Have you tried removing the OWC RAM, and just running with the factory-original RAM a day or two?

If not, I strongly suggest that you do this...

Thanks for your reply (you beat the rest of the world)

I did not try that but I was thinking about it, have you had experience with faulty OWC memory?
Over the years and all machines I had one defective stick from them, it came a lifetime warranty but since the machine (MDD G4) was already getting old I did not bother, besides living in Belgium would make it costly because OWC's warranty did not does cover one of the shippings (I do not recall for me sending the defective ones in or them for sending me the replacing one's under warranty)

As I speak the iMac has been running for more than a day now without a hitch and 2 user accounts logged in.
Strange....

 

[JD2]

As Fishrrman suggested I tried to start the iMac without the 2 8G sticks resorting to the original 2 4G sticks.

And it seems to be doing ok now, I have left the remaining stick in their possition being the second slot pair.

I even have to admit even with 8G under the hood my late 2012 iMac runs quite "snappy" for my day to day use.

Before I tried the suggestion I tried 2 other things:

Starting the iMac in safe mode what seemed also to improve the situation: no crashes at first but after a hours I did have yet another crash.

I also tried starting the iMac from an external bootable drive but this also resulted in crashes.

So Fishrrman thanks again for the suggestion, I am going to try to have the lifetime warranty working for me.

JD2