'Lamp Defective' Brake Lights

[samali1999E320]

The message 'Lamp Defective' came up and when I checked it was both the brake lights that did not flash when the brake pedal was pressed, but the center brake light flashed ok.
I tried wriggingly the brake switch and it solved the problem for about 5 miles of city driving and then came back again.
I tried again with the brake switch and it was ok for a while and again the warning came back.
Any ideas what could be the problem.
I thought if the brake switch was bad then even the center brake light would not work.
Thank you anyone out there who could give some input to this problem.

Sam