Is this the M113 engine? If it is I had EXACTLY the same problem with a 97 C240 with the M112 sister engine... they are exactly the same apart from the number of cylinders. It would start fine hot and cold but require prolonged cranking after about six to seven hours of standing.

I went through a whole process of replacing bits and pieces as there were no fault codes thrown up.

Replaced Crank positon sensor and spark plugs. What finally cured all running problems though was a new Mass Airflow sensor!

Good luck

Doug