On my '93 300E 3.2 (best car I ever owned, over 360K miles) the intermittent crank but no start was either the OVP or the fuel pump relay behind the battery. IDK which it was because I threw both of them at it at the same time and the problem was gone. Hope it is that easy for you!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer