OSX wont save external monitor screen arrangement settings, weird graphics issues with displays Ive got a 27" iMac 4.2ghz i7, 16gb ram, 8gb GPU, running OSX 10.13.6

Ive got 2x external 1080p monitors connected to it via the thunderbolt ports using 2x of these HDMI to thunderbolt adaptors

Recently Ive been having allot of issues with all 3 screens, mostly to do with the order in which they are setup in system preferences &gt; display &gt; arrangement. I have the screens setup as follows on my desk from left to right : 1080p display, iMac 27", 1080p display. I originally arranged the screens like this in the arrangement tab, but every time the iMac goes to sleep the arrangement seems to randomly re arrange itself, so each time the computer is turned back on i need to reset it. 

Yesterday i was having allot of issues with all displays, the iMac was displaying the image below (see attached), the far left screen 1080p wasn't working, and when it did it the other 1080p and the iMac screen would dim very dark. I swapped out the HDMI to thunderbolt adaptor, and it seemed to get all screens working, but they are still not saving their arrangement settings. 

Any ideas what might be wrong here and how i can fix it ?
That looks like a GPU failure and not any sort of software issue.

Have you tried making a new user account or does this garbled text happen at the pre-log in / pre boot screen?

Worst case, you would need to bring a second bootable OS to that Mac to be sure it's not the OS if you get clean graphics in the startup mode, but garbled once the OS takes control of the GPU and enables the standard rendering options.

If you can work with it normally before the image fails, make a new user account and make sure that new account has the issue before going to the trouble to make a second OS or erase/install.