What's that? iMac screen shows strange pattern...

[rin67630]

Sometimes my iMac shows a strange pattern for a few minutes uppon booting an overlay looking like old analog CRTs with coarse lines in front of the normal desktop that slowly disappears.
Has someone also experienced this?

 

[rin67630]

here a detail picture:

The coarse vertical lines are NOT the screen pixels which are much finer.

 

[Fishrrman]

What year was the iMac made?

Looks like either a display or discrete graphics failure.

Depending on the age, might be time to start shopping for a replacement.