Dell M782 -display image

[NWCook]

I recieved a second hand Dell M782 monitor from my cousin. ( I believe it is a 17" (16" viewable) shadow-mask with .25 dot pitch). I did download and install the monitor drivers from Dell's site. However it seems the display image is always moving/changing size slightly on a constant basis. I usually have to adjust the monitor setting several times a day to get it centered. Does anyone know what could be causing this? Also, is there a really big difference in image quality between this monitor and one of the NEC Diamondtron people keep raving about?