Could be as simple as worn ignition system components - spark plugs, distributor cap and rotor. Over time the current transfer surfaces in the cap and rotor develop scoring and pitting, reducing the voltage that is delivered to the spark plugs. If the cap and rotor have over 30K miles on them, pull and inspect; invariably they'll need renewal. As for the plugs, I believe M-B calls for inspection/renewal every 15K miles.