I've noticed this since I bought my car.
Sometimes the engine feels so strong and the throttle feels like it had a telepathically direct connection to the engine, at other times it feels so sluggish.
Its got nothing to do with outside temperature as this inconsistency can be felt even between 2 engine starts on the same day. Sometimes the engine feels better during a hot day (33-35'C) than on evenings (27-30'C).
I can't seem to locate the source of this. Sometimes my idle isn't too good (takes its time to come down) but can that affect engine performance? Also noticed recently that my water temp fluctuates more (sometimes higher, sometimes lower).