I had a friend with a Space Invaders (not II) with a similar problem. We separated the two boards from each other, cleaned the edge connector with some alcohol and put it back together. That took care of the problem. Over time the two boards had separated from each other a bit and weren't making a good connection. If your board is the same 2 piece 'L' configuration you might give it a try.
Looking online it appears the color versions may be two boards on top of each other connected with ribbon cables. In that case you might try to carefully remove and reattach the cables.