My IMac has coloured vertical lines on startup eventually changing to a blank screen
Sounds like a failing graphics chip. Time to make an appointment at your local Apple Store or AASP to have it looked at.
i have similar problem with macbook pro. https://discussions.apple.com/thread/4530266
but after resetting NVRAM (I followed the instructions from here http://support.apple.com/kb/HT1379) it runs normally, and appear from time to time now.
is it graphics chip, rkaufmann87?
What year model imac do you have. If resetting the PRAM / NVRAM according to the article and displays ok then yes get it to an AASP or ARS to have a video stress test done.
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