[old_tv_nut]

This could be a tricky question. Turning the brightess and contrast down may be making the dot last longer, but with reduced intensity. If so, it's like braking your car gradually from high speed - raises the brake temperature much less than trying to stop quickly from high speed.

Did you try it without reducing the brightness and contrast at first and then decide to do that? If so, how quickly did the spot go away with the brightness and contrast at normal settings?