Dead spot in my pedal stroke

I had my tri bike fitted last year… Anyway, i’ve made some modifications to make it “less” aggressive (I know… This is the opposite of what most triathletes seek… however, I was REALLY aggressive… think Steve Larsen) Anyway, on some of my early season outdoor rides, i’ve noticed that when i’m spinning out in a ridiculously easy gear after intervals to flush the legs, there is a serious jerk in the bottom half of my pedal stroke.

I know the obvious answer is go get re-measured. However, I was wondering if any of you had some early diagnosis… Is my saddle too short? Maybe I just have a horrible pedal stroke… However, I spun all winter (on a spin bike mind you) and really worked on a circular pedal stroke and one legged drills.

Any suggestions would be much appreciated… Thanks