Graphic Issue when editing vertices for a large model.

Hello been using blender for awhile but have got into larger models and I came across this issue. When ever I try to edit the vertices in solid model the window is filled with artifacts, lines, colours completely obscuring the model and making it very difficult to edit the model.

My computer is a duel core, windows xp sp3, Ati X1900 256 graphics card with up to date graphics drivers, gig of ram. If anyone can help solve this annoying issue I would be grateful.