It sounds like there is a bad connection in the indicater itself that is intermitently loosing connection. Is there corrosion in the unit where the bulb locates?
Take it off, take it apart and check all connections.
If this doesn't cure it, remove both front indicaters and swop them over, see if that cures the problem or it should give you a better idea of where the issue is.
I think it's just a bad connection on the faulty indicater.