Force feedback uses a big motor to knock the steering wheel from your hand. Resulting are a big spike on the power supply, and a strong magnetic field as the motor ramps up. Both can really interfere with your lcd.
Step 1: Play with the position and shielding type of your vga cable to see if there is any magnetic coupling ruining the high bandwith VGA signal.
Step 2: Check the power supply. As you are using a seperate computer supply, the spike could come through the ground line.
The vga cable interference possibility is most likely, though.