Dell :: Inspiron 6400 On External Monitor, Regions Of Image Flash

I have been using an external LCD monitor hooked to my inspiron 6400 for 6 months, everything is fine. but 2 days ago when I moved the laptop a little far away from its old place,

I tried to add a 6-feet VGA extension cable to the present VGA cable of the external monitor . the image with extension cable is horrible, so I take the extension cable off and move the laptop back to the old place. and a problem showed up:

when I hook my inspiron 6400 to the external LCD monitor via VGA port on the laptop, some regions of the image on the external monitor flash (if I put on a document, the words are shaking, or "dancing"),

for example the upper right corner, and the middle section of the screen are doing this right now. and if I take the VGA cable off and put it back on, the flashing region changes. but one thing for sure: Not all the screen flash, only several regions.