Strange XFree86 problem

The system is a Packard Bell Model 755 with SiS 5598 on-board video
using shared RAM.
The problem is the mouse position indicator (X, arrow,etc) is 2 times as
far from the top
of the screen as the window manager thinks it is. When Xfree starts up
the X is at the bottom
of the screen and will go off the screen if you move it in that
direction. If you move the X
to the middle of the screen, and then try to move it off the sides or
top it stops at the edge of
the screen. If there is an xterm window at the top of the screen, it
will stay "active" until the
position indicator is moved down to a position that is twice the height
of the window below
the window. If it is moved to the side the window goes inactive as the
position indicator
leaves the window.
Has anyone got a cure for this, or can shed any light on the subject?

Thanks
Warren Steffen