Quoted from markmon:
I would start in switch edge test. Hitting the left flipper button should trigger two switches: lower left and upper left. My guess is you are only getting one and the video mode requires the one you don't get. Next would be to examine / clean / fix / replace the flipper opto board responsible.
I had the same problem on my STTNG, only one opto was reading in switch test. The other opto jarred loose from the opto board and needed repair.