Starter Solonoid or Battery?
 

Sometimes I turn the ignition and get nothing. This happens several times before the 1991 560SEL finally starts, turning the starter slugishly. I complete my trip, turn off the engine, then reignite again immediately without hesitation. The radio and clock reset as if power was lost. Does it sound like solonoid or battery?

It sounds like the Neutral safety switch located on the transmission.

http://www.peachparts.com/shopforum/diesel-discussion/191207-starter-problems.html?highlight=safety+switch