The problem has now finally been resolved (or so it would appear). The main reason for the poor running and intermittent lack of power turned out to be the distributor caps and contacts (as indeed suggested initially by MB DOC). The ignition leads were also replaced.

The 140 was unpleasantly heavy on my wallet in September ;-), but it has been returned to an immaculate running order.

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Mateusz
1986 126 500SEL
1986 126 300SE
1992 140 500SE
1996 124 E200T
1999 210 E320T
2017 205 C200T mit Schaltgetriebe!
2010 639 CDI 3.0
2008 204 C180 KOMPRESSOR