Key works sometimes in ignition. c230k

Hey Fellas,
Newcomer to the world of Mercedes. I bought a pre owned 2000 c230k and it worked well until last week the main key would not work in the ignition as before. I have replaced the batteries and the keyless entry,trunk and panic features work. I use the flat emergency key to start the car. The main key works sporadically no rhyme or reason...
Thanks for your input.
Alex.