If there was an ignition problem there would be backfiring as the unburnt fuel in the exhaust system ignited. Maybe your fuel pressure is dropping. Does the problem happen in every gear at a certain RPM?

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Tony H
W111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux Pickup Truck-Dad bought new
'73 Toyota Celica GT