Won't start sometimes

What is your question?

I have a 2002 Mercedes CL 55 .I am having a problem with it starting up sometimes .If i leave it sitting for a day or more ,It has to be jumped off .it cranks right up once the jumper cables are attached . My battery has been check ,my alternator and starter both are putting out good power ? What could the problem be ?

What makes this problem better or worse?

Worse when it’s cold

How long has your 2002 Mercedes-Benz CL55 AMG had this problem?

2 months