Maybe I was a little too quick with drawing conclusions.
Today I had a 30 mins drive somewhere and everything seemed fine. A couple of hours later on the way back the car would not coast at 750 rpm but did so at 1000 rpm. And sure enough, at the next traffic light it idled at 950 rpm. Just like it did on the other ECU.
Turning the ignition off and then restarting the engine restored the 750 rpm idle, and it didn't misbehave on the next trip.
So maybe the symptoms have alleviated somewhat on the correct ECU, the actual cause must still be there. Bummer...
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