Just as a test, try lowering the resolution a couple of notches, and avoid any of them listed as "stretched". See if that improves anything.
I have a late 2006 Macbook Pro hooked up to an Asus VE247 24" monitor.
I've been having problems with the monitor refreshing the screen.
For example, when I am using photoshop, it takes a while for me to see
the results of whatever button I pressed (aligning two photos as one
example). Or, if I press "Undo," it doesn't show me what I undid until
I click on another button to do something else. Also, in Firefox, when
I open up a menu (the File menu for example) and mouse over a menu
choice with an arrow next to it, the rest of the menu doesn't come up
until I click on it.
I never had this problem until I hooked up the external display.
A little background about the monitor (from System Profiler on my Mac):
Resolution: 1680 x 1050 @ 60 Hz
Pixel Depth: 32-Bit Color (ARGB888
Mirror: On
Mirror Status: Hardware Mirror
Online: Yes
Rotation: Supported
And about my macbook pro:
Resolution: 1680 x 1050
Pixel Depth: 32-Bit Color (ARGB888
Main Display: Yes
Mirror: On
Mirror Status: Master Mirror
Online: Yes
Built-In: Yes
Also, the refresh problem is also happening on my macbook pro screen when I have the external monitor plugged in.
How can I fix this? Please help. Thanks in advance.
Let me know if you have any questions or need any more information.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic