Answer to: How to fix "vertical lines" on my iMac 27 late 2009HalelujaH! FIXED!!! I got a free 27" IMAC because of this problem! Fix was simple! Boot from MAC osx Snow Leopard CD. Erase and repartition internal hard drive. Install MAC OSX Snow Leopard, then install additional software from folder on install CD. Once install is complete the bars went away! Presto! OSX Snow Leopard Install Disk was ordered from Apple for less than $20.00. iMac was donated to local computer recycler that only accepted PCs. So they gave it to me. Do not upgrade these machines beyond MAVERICKS! God Bless and God Speed! Professor out.....TZZZZSSSSSSSSsssssssssssss.