"sliding" overscan ?

I have windows 98SE, using the ATI 7500 and the viewsonic 22" LCD. If
I use the DVI connector, a funny thing happens - windows outputs a
resolution HIGHER than what i set it at, so that I have to use the
mouse and push against an edge to see all of the screen. For example,
if I set the rez at 1600X1200 (the native rez of the LCD), then windows
98se outputs 1920X1400 (or something near that), because I only get
about 85% of the desktop visible - if I move the mouse to the right
side of the visible screen, then the whole desktop will move and I can
bring the rightmost part of the desktop into view. Ditto for the
bottom. It is like windows outputs the desktop at 1920X1400, but then
windows clips whatever portion of it that will fit onto 1600X1200 and
outputs that, waiting for me to use the mouse against an edge to move
the visible window around. This is very confusing, because, as I
change the rez (in the settings tab), the amount of "sliding oversscan"
changes and at 1024X768, overscan goes away entirely. This is most
confusing to me because windows knows my monitor is the viewsonic
syncmaster (22" + 1600X1200). Also, if I use a different computer with
a cheap built-in video chip (intel something or other), then this
problem goes away.

Also, and this is MOST confusing, when I use the 1024X768 rez and make
the 'sliding overscan' go away, a funny thing happens when I start my
ATI TVWONDER output - the "TV Window" is clipped by about 15%. If I
change monitors (to an old CRT), then this problem goes away.

I must admit that I am stupmed as to the curious behavior of this ATI
card and my LCD using the DVI connection.

Is there a secret "trick' that will let me fix this and go ahead an be
able to use my DVI connection on my LCD and not get any of these crazy
overscanning issues?

Thanks, littleberry