Ian,
Do you have the latest video card driver for your machine? If not, please get and install this and see if the issue goes away. If you have the latest driver, then can you post a screen shot of this happening and a small VI that reproduces the issue somewhat regularly? It would also be helpful to post what operating system you are using as well as what version of LabVIEW. Thanks.
Randy Hoskin
Applications Engineer
National Instruments
http://www.ni.com/ask