You should look at a new vacuum pump . If the vacuum pump bearings are worn it will fail at the most inconvenient time and take out the timing chain with it. When you say the timing device has no obvious wear I'm not sure if you mean the surface where the timing pump rides or if you have disassembled the timing device to check for wear in the advance mechanism. The weights could be clanking around in there.

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'82 300D - Light Ivory, 2nd Owner (Back in the wind April 2013!)

'95 E300D - White, grey interior. (Suffering from stuck/broken glow plugs)

Deuteronomy 22:4-
"Thou shalt not see thy brother's ass or his ox fall down by the way, and hide thyself from them: thou shalt surely help him to lift them up again."