One possibility is JP3, near the middle of two VGA sockets. This is the 75 Ohm termination for the VGA out. This is normally open and assumes the monitor is terminated (most are). If this has a jumper on it, and your monitor is normal, this might mess up the picture, somehow (I'm no video engineer).
The jumpers are:
JP1 - Slow Buster mode - normally open, for Buster chips older than Rev. 11. The manual says it only works for boards with serial numbers with 81xxxxx.
JP2 - reserved - normally open
JP1 and JP2 are between the QFP chips above the Zorro connector. JP1 is towards the front of the computer, JP2 is towards the VGA sockets
JP3 - Output termination 75 Ohm - normally open
Near the VGA sockets
JP4 - Input termination 75 Ohm - normally closed
This is at the bottom of the VGA In socket
JP5 - Reserved - normally open
This is on the bottom edge of the board almost halfway between the Zorro connector and the VGA In socket.
