Carb ice
I had the engine in my Robin miss a beat the other day when in the cruse , when I went to push the throtle FWD a slight resistance was felt and it missed another beat.
I think that this was the engine reacting to the ice that I had disturbed by moving the throtle.
Carb ice may seem unlikely with the heatwave that we are having at the moment but I think that one should remember to use the carb heat as even with a big temp/dewpoint split the air contains a lot of water at these high temps.