one of my screen's response is so slow.

[rich.bae]

Hi everybody.
I'm using dual-screen.
one of monitor is conntected with d-sub connector.
and another is connected with dvi connector.
but the d-sub connected screen's response is so slow.
I mean that whenever I click main menu, sub menus are appeared so slowly.
well, dvi connected screen isn't do like that.
I think it's not the problem of d-sub or dvi connecting.

FYI, My h/w specs are like below.
CPU : Intel Core2-Duo
VGA : nVidia 8600 GT (have two connector. one is dvi, another is d-sub)
RAM : 3G
HDD : 500G
Monitors : 21(dvi), 17(d-sub) inch

and i use Ubuntu 8.04(Hardy)

Is there everyone who already experienced like me?
then, how did you solve it?

thank you a lot.

P.S. Sorry for my poor english. ;-)