Screen flickering and horizontal dotted line on screen of my new iMac.

I have a new iMac for 2 moths.

This morning sudenly my Mac shows a horizontal dotted line (1 pixel), at the bottom of the screen, at 15% from the bottom.

Below that line, the image starts to flicker now and then.

There is also a vertical yellow line accroos the screen (1 pixel)

The flickering is very anoying, what can I do?