Date registered: Jan 2007
Vehicle: 1998 SL500, 1959 220S, 1970 280SL
Location: Falls Church, VA, USA
Mentioned: 5 Post(s)
Quoted: 94 Post(s)
The first thing I would do is put one of the "failed" ignition modules back in and see if the car starts. If it doesn't, put a timing light on a plug wire, crank the starter, and see if you're getting spark.
I personally think you have a flakey fuel pump relay. If you get in a no-start situation, have someone crank the starter while you put your hand on the fuel pump to see if it's running.
Falls Church VA