i used to have the same problems u described but i have a E320 and for me a new fuel pump relay and ovp cured everything

it was just strange, u go for a drive.....stop by a store or whatever real quick (under 10mins)....return to the car....cranks forever....nothing.....then wait a bit....starts up up fine as if nothing is wrong

ur car might not have any problem during cold starts but eventually for me just driving the car was a crap shoot....i'd never knew if it was gonna start in the morning or worse leave me stranded if i dared turned off the car

good luck on ur problem and do keep us posted

the 420 is the back up plan if i can't find the e500 :-)