Chud,
Just went through almost exactly the same thing with my TAF.
Set it to single lamp test, scroll through until you have two lamps flashing.
Now go through and one at a time, remove each lamp in column 8. My guess is somewhere along the line things should all of a sudden go back to just the one correct lamp flashing. That lamp that you have pulled should be the cause of your problem.
Might be diode, might be wired wrong, might be shorted lamp, but at least you have narrowed down your options.
Give me a shout if you need any more help. I just had the same problem and it ended up being a "phantom" trace on a light board.
Good luck!