Q My 24” iMac has developed a display aberration when starting up. In this a 3 mm thick bar appears across the width of the screen about a third of the way up. Switching resolutions fixes it, but the same happens next day too. What is the cause?
A If this only occurs with frequency when starting up from cold, but rarely after a restart, it is most likely an early sign of hardware failure affecting the graphics card. That can be repaired by an Apple-qualified engineer, but is likely to be expensive relative to the value of your Mac.
If equally frequent after warm restarts, it is more likely to be a software conflict, from an old or incompatible driver or other extension.
Updated from the original, which was first published in MacUser volume 30 issue 11, 2014.