Curious that you did not blue-screen.

My initial thought was heat since I started having similar problems when the weather got warm and my computer was flat on the desk. Tilting it up so heat could dissipate solved that problem.

The only non-blue screen shutdown I have experienced was when I upgraded my RAM and did not seat the new ones properly -- but that happened immediately. Is is possible one of your modules be seated properly while the other(s) is/are not?

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black