Again, take out the light board. couple screws, no problem. Follow the traces from the misbehaving light and determine which pin feeds that circuit. Examine the solder holding the pin in the board to see if looks old and maybe cracked. Regardless, while you have it out, heat up your iron when hot enough to melt solder, gently touch that joint to reflow the solder. it will go from a matte finish to shiny and then will dry somewhere in the middle. Inspect the solder at the light for divots and reflow those as well.
If there is another board with the same amount of lights, Pull that one off and switch boards. If the problem is there in the board in question at the new location, it is the board. If the problem is at the old location where the new board is, it is not the board.
"You can do it!" (waterboy)
Running out of motivation quotes. C'mon, get your head in there and fix it. Just take your time.