Before hauling a heavy computer to an Apple store, make sure you know how old it is, or the exact model. Apple only works on computers made in roughly the last five years, and the iMacs in this forum--G3, G4, and G5 models-- were all made prior to 2006. The description of the vertical lines sounds like a G5.
This article explains what Apple does and does not fix:
Here are two articles to help you ID your model: