Opossum, make a note of your rpm vs. road speed next time it takes off feeling peppy - before it shifts. Note the same next time it takes off sluggishly. I bet you'll find rpms much less for the same road speed - i.e. you're in second gear.
Note too, if you shift into the lowest gear while you're in motion, it won't shift into 1st. If you do so while stopped, it will shift into first - and stay there.