The reason this dead-spot wobble occurs at the top of the pedal-stroke is that you are reversing the torque on the arm. On the way up, the crankarm is actually pushed backwards by the dead-leg. That is, the arm is "trailing" the spindle since torque through the spindle is "pulling" the crankarm up to lift the left. Then at the top, you sequence reverses and your leg "pushes" on the crankarm and moves it into a "leading" position where it's ahead of the spindle.
The "dead spot" you feel in your foot is due to the clearance between the crankarm and spindle (loose crankarm). It's like a woodruff key that's going into too wide of a slot, depending upon the direction of load, it will slide back and forth inside the slot until it bumps up against the stop at the other side of the slot.
Last edited by DannoXYZ; 05-24-09 at 06:28 PM.