I see this problem all the time. It was an issue for me in WF 3.0, 4.0, and now 5.0. In fact I just saw it this morning. Usually what I do is unhide the layer, refresh and thenhide the layer again and the items will disappear. If you find out what's going on, let us know. It seems like it shouldn't be a graphics issue but FWIW I have Nvidia Quadro FX 1400.