Problem starting clk55 amg Please help anyone
 

Hello everyone, i am new to owning my first mercedes let alone this 2000 clk55 amg.. i love it, had it for about 6 months now but unfortunatly i have a slight problem..
I didnt drive my benz for about 1 month before i took it for an M.O.T i got it there and it passed straight through but then thats when the trouble started.. well it didnt start up. After a couple of hours and a call to the RAC we found a very fraied earth cable so we fixed this and managed to get lights up on the dash :) progress!!! But no start-up, didnt even turn over.. After another hour or so we discovered that by running a direct live to the starter motor she fired up :) so what could this problem be???
Sometimes it will work off the key now and sometimes not.. as you can imagine this needs to be sorted as i cant keep lifting the bonnet to start it as im sure its doing it no good.
Any Suggestions will be much appreciated,Thank you very much for your time taken to read this