Engine icing problems
It's a long while ago now but when we first got B737-200s in BA I was operating to one of the Greek islands as a very new command. Somewhere over the western Med in the cruise in clear air the airspeed started to fall back rapidly with the thrust levers well up. In desparation I switched on the engine antice and the speed was quickly restored. When I reported this later I was told this was a known characteristic of the PW engines and didn't merit further comment! Plus ca change...