Display went pixellated with bottom left to top right diagonally squiggly reddish lines. Problem persisted for 3 days. Target disked it to another Mac to backup. All data, files, etc. OK and backed up. Turned it off, rebooted and screen returned to normal. Been working fine now for 2 days. Notes: Added SSD drive about 3 months ago with no problems. Optical drive had been bad, not mounting, reading, writing DVD's for about a year. Motherboard replaced about 2 years ago. Any ideas what happened and why? Likelihood of it happening again? Preventive or diagnostic measures? Thanks!