In a Williams Flash I have 3 incandescent playfield GI lamps which come on at full power when the game is powered on, but fade to very dim after a few minutes of play. What would cause lamps to work properly at first and then fade over the next 5-10 minutes?

No common ground, new bulbs didn't fix... two new sockets seem to have done the trick. The third socket is more difficult to reach and will be my next effort.

Thanks for the suggestions!