I noticed that whenever my left foot goes through the 1 o'clock position (looking from the left side of the bike) I feel something like a very, very small, "skip" in the pedal rotation. The skipping occurs always in the same spot whenever I'm pedaling. At first I thought it was a loose BB or damaged bearing. However, if I spin the cranks by hand I don't feel any skipping. Someone suggested that the culprit might be my rear cassette or an uneven pedal stroke. Any comments?