I have a similar problem on my laptop. However, I don't use a screen saver, I just use the power management settings in XP to turn of the screen.
The power management kicks in and turns off the internal screen and the external monitor and then it immediately wakes back up sometimes. Once it does that, it continues to do that until I "fix" it.
I don't know why it does that, but I find that it helps if the mouse cursor is on the internal screen and not on the external monitor. The other thing I've done is changed the power management settings to turn off the monitor after 1 minute to test it and see if it's working. Sometimes just changing that setting and hitting ok fixes it. Once it's working, I change the pm settings back to 10 minutes.
If none of that helps, you can try googling for your specific issue.
-David