The company I work for builds a product that is controlled by a PC built
into the system.  The operator controls are displayed on the PC monitor.
We've had a recurring problem on all of our machines (probably a few dozen)
that run windows 3.1 (in standard mode) in combination with a realtime
operating system called RMX.  The cursor will infrequently disappear from
the screen with no apparent response available from the trackball pointing
device.  The keyboard still functions when this occurs and while navigating
via the keyboard the cursor will, sometimes, eventually reappear.
The Logitech trackball is a PS2 device.  The problem has also been seen
with a serial mouse (different IRQ).
The basic PC system used in our product is;
10 slot backplane industrial PC case
Intel 486-100
8 meg RAM
Diamond video card (standard windows vga driver)
many add-in cards at various memory addresses
Logitech trackball

Any suggestions will be appreciated.