Is there anything electronic on the inlet manifold or on the throttle body? Is it possible that leaning on the air filter box is flexing some electronic connector juuuuuuuust enough to make connection properly? Lord knows I've chased down more than my fair share of electronic gremlins in my cars. Any time I have flaky behavior, the first thing to come out nowadays is the soldering iron!

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]