You are losing your video signal, or the monitor does not recognize it. Make sure you only have either the DVI cable or the Analog cable connected, not both. If using the DVI check your connectors and pins make sure they are secure. When this is occurring is the signal light flashing from 1 to 2 and back again? Try the analog connector, again check all the pins and make sure connector is secure. Check you have the refresh rate set to 60Hz, and resolution to optimum at 1280x1024. If this continues, reseat your video card, and reinstall your video drivers. Follow Dell-ChrisM FAQ for video driver at the top of the Video Forum.
I'm having the same problem with a 1901. The cables are tight using the DVI. PC is a 4600c so it only has DVI out. Any other suggestions? This was a repalcement for the customers last monitor who had pixel issues.