**Please note that resetting preferences will remove all the custom settings and Illustrator will launch with default settings. You can also take a backup of the folders in case you want to. Location is mentioned in the article.
1) Is it happening with a certain kind of objects or with specific objects and not others of a certain kind, and if either what can you see about them in the expanded Layer and in the Appearance palette when selected (you can release the Clipping Mask to get to them)?
2) If nothing obvious appears from 1) is it only happening in a particular document? If that is the case, is it also happening if you move the artwork to a new document (you can try first with everything at once, and if it fails, in portions and see when/if it fails)?