Users always feel frustrated when they see horizontal lines on the screen and unable to do anything. At times, it seems like a big issue to resolve but there is always solution for everything.
First, you should ensure that if it is loose cable or connector to check if it is cable’s or exactly monitor’s issues.
When you see horizontal line take a screenshot and see carefully if loner are clear screen of course. When you see lines and see the horizontal lines are missing then it is clear that GPU or any internal circuitry issue.