Out of date video drivers and OS packages are known to cause problems when
using fullscreen mode, so check for updates. You should have at least Windows
98, DirectX 6, and recent video drivers (or MacOS 9 or DrawSprockets 1.7.2
if you use MacOS 8). Under MacOS, G-Force has been shown to conflict with
Kaleidoscope and OpenGL 1.2.1. Also, try deleting your "G-Force Prefs" file
in the case that it's become corrupted (there's an endless number of ugly
things that can happen from a damaged prefs file). See the following question
to generate a new prefs file.
When you start G-Force, it scans the "Sprites" folder for text,
images, and movies to add to the particle slide show. If you're running
MacOS 8 or 9, it's possible G-Force is running out of memory as its loads
the items in this folder. To solve this, you'll have to allocate more RAM
to your audio player (via the Finder). Since G-Force is typically run alone,
try allocating half of your total free RAM to G-Force (half because G-Force
stores sprites in MacOS memory space, not in its own memory space).