Mouse Settings

When I try to look at a map, either MapQuest, AAA maps, or Google maps, I can not hold the map still. It magnifies, then opens up and moves all over without my touching the mouse. Otherwise I have no problems with the mouse making strange moves. It has reached the point that maps on my iMac are impossible to use. Has anyone an idea what the problem would be? It is a wireless Magic Mouse. I have slowed the Mouse movement in the pref panel but that is no help.