IMac screen problem

It's hard to say simply based on that description.Are the lines static and always in the exact same place? Do they persist between reboots? Have you noticed any other performance issues or crashing? What iMac do you have?

 

If it's new, return it. Definitely. It's likely a video card issue, or it could be the connection between the screen and logic board. Either way, it's likely an expensive repair.

 

white streaks

I had white streaks emanating from the top corners of my new iMac 24. It turned out to be defective glass, they replaced the glass, and all is good. The white streaks were part of the glass, no way to remove them.