The bar drifts well forward of midfoot as the set progresses. This is probably related to the fact that you are not in control of the weight at all on the descent. Either way, make sure the bar is above midfoot on each rep.
Setting your back should be done as a single deliberate action. You are "running up" to it with two or three tries and only pulling on the third "run", and so your back is not fully set when the pull begins. Set your back, take a deep breath *without moving*, count "one one thousand" in your head, then begin the pull.
Do not wait so long between reps. Again, lots of wiggling and huffing and puffing which is wasted energy.