OK, here's the deal. I have fixed this 8 or 9 times so far. You did all the right things. One more step: the metal contacts on the plastic bulb holders get flattened out over time. I used needle-nosed pliers and GENTLY bent those silver tabs back outwards so they can make good contact with the other contacts. This should do the trick..
"Patriotism is the last refuge of a scoundrel." -Samuel Johnson