S320 won't start
 

Hi
i have a 2002 mercedes S320. i've been having a problem starting up the car specially in the morning. i suspected that the battery could be dead but i just replaced it with a new one like 2 months ago. When i put in the key & turn it the radio & everything turns on but the engine won't start!
I found a temp solution which is take the black wire apart from the battery & wait for a while & put it together again then the engine starts but i'm not planning to do this every time i want to drive.
What could be the reason for this?
The other problem which might not be connected to the one above is that sometimes when i start the engine in the morning (winter) the radio changes channels by itself & the sound volume goes to max by itself. what is the problem ?