Are you sure your game board is working? If another deflection board does the same thing, that makes me suspect the game. If you set your meter on AC voltage and measure the X out, Y out and Z out see what you get - zero means bad game board output.
I'd also turn down the brightness until you get it sorted out!
Note that there are different revisions of the G05 deflection board, and have different pinouts at the keyed connector of P100. If you're swapping boards make sure they're the same rev.
See the B&W vector monitor FAQ if you haven't already.