Had a similar problem a while back with a World Cup (same era) where the left pop bumper stayed energized and fried itself. Ended up replacing both transistors to that solenoid (the TIP and the 4401) on the driver board and got it to work. I recommend checking those transistors if you haven't done so already.
Link to my post: http://pinside.com/pinball/forum/topic/help-with-williams-system-3-world-cup-not-playing