A poor connection at one of the sockets, or possibly a bulb that's fixing to burn out. Good idea to check the all. I use the highly technical "flick of the finger" to test the filament. Bulbs fixing to go will usually break the filament under the flick test.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.