If ye could, try working on a different map for a little while? It could be the poly-count, if I am remembering the term correctly, that is starting to glitch y'er screen. It may be a certain mesh y'er using as well, that might be triggering the glitch.
M'recommendation, besides what these two lovelies before me 'ave suggested, is to try editing the Watermill map to see if ye get the same results. Best of luck to ye!