I am using a second flat screen monitor with my Dell Inspiron 600M. Every time the laptop goes into sleep mode I have to reset the resolution on Setting, under display on the control panel. Sometimes I get a message on the flat screen saying reset the display to 1440x900. Other times I just get a partial image on the screen and have to change the resolution a few times.
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Sounds like a driver issue. Do you have the latest ones?
You can always choose hibernate over sleep - generally it's more driver-friendly.