The logic is simple. Temporarily disable all of your startup extensions. If the problem goes away, then you know you have a startup extension conflict. There are three different ways to disable your startup extensions: hold down the Shift key during startup, use a startup management utility, or remove all startup extensions from your System Folder.
To quickly disable all of your startup extensions, nothing is better than the Shift key method. It is simple, easy to do, and guaranteed to disable almost all system extensions and startup extension-type control panels in your System Folder.