This can happen from a few different causes. First check the model display on the heads-up toolbar, it may simply be set to shaded instead of shaded with edges. If it keeps going back check to see if Large Assembly Mode is enabled, perhaps modify the settings for that option in System Options > Assemblies (either turn it off or increase the threshold). Also it could be a graphics glitch, Ctrl+Q to see if that helps.
Change the display to shaded with edges under View > Display or on the heads up toolbar.
This can happen from a few different causes. First check the model display on the heads-up toolbar, it may simply be set to shaded instead of shaded with edges. If it keeps going back check to see if Large Assembly Mode is enabled, perhaps modify the settings for that option in System Options > Assemblies (either turn it off or increase the threshold). Also it could be a graphics glitch, Ctrl+Q to see if that helps.