Did the shop scope the ignition system? This engine does have resistor wire ends that do fail often. Also have the valves been adjusted lately? This is a solid tappet engine and the valves need to be adjusted every 15k miles. If the problem is on the fuel side, have the fuel distributor checked.
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM