Update: It was the cable adjustment. I realized that the freeplay on the blade control was quite excessive and correcting it needed all the adjustment available on the cable. Since this has been occurring since I bought the mower ( and slowly worsening), I suspect that it was never adjusted at the factory. Chapter two of this episode was that the engine started surging badly at idle. I decided to take the whole hog to task cleaning the carb and jets, gas cap vents, filter, gas lines, replacing the air filter and putting new gas in. Don't know which of the culprits it was but it runs great now.