]The monitor needs to warm up. The image will flicker a few times and the brightness flickers as well. In fact, it looks like I cranked up the brightness or contrast. After 15 minutes it stops and the blacks return to blacks and the image stays put[/li][/list]
If it's an old monitor, try turning the brightness and/or contrast up and down a couple times, then adjust to suit. It could be some crud in the control that keeps it from making proper contact until some heat from the chassis gets to it. Do the same thing with the HV control on the flyback, but don't crank it, just turn it up and down a little. Outside of that, it could be that one of the caps, or other components, just needs to warm up before it starts working properly. As a side note, you always want to let a CRT warm up about 30 minutes before adjusting the picture. Nature of the beast.