@mikahl post some pictures showing us what your monitor displays. Horizontal lines are commonly caused by LCD panel issues. If you already have disassembled it, post some pictures of all of your boards as well. Try to connect a different input source to the monitor, see if that changes anything. The concern I have is a bad main board or bad LCD drivers.
[guide|21499|Adding images to an existing question - iFixit Repair Guide|new_window=true]