I have a similar problem with good mornings. On the first rep, I can barely get to 15 degrees. It is not until the 3 or 4th rep that I can get to 90 degrees. This happens for every set.
Well i guess what raptor said lol.
That's because that movement is not engrained and automatic. You need to improve the movement efficiency for that movement (more volume and or higher frequency).
ah man yeah makes sense. damn nvr thought i'd have ME problem. that day did 80x5 might've progressed too fast as i was stuck about a sec after i bounce up a little from the bottom of my squat range. the movement efficiency could've played a role. Btw, i remember reading a part of your journal where you said you have problem with a certain weight as you were quad dominant and not used to the hip drive or something like that? I think i have the same problem. i tried driving up my hips, had to use some quads or god knows what muscle i did to go back up, while probably going out of line of the supposedly straight movement of the bar vertically.
I have the same problem sometimes at my top sets , only at bench press and full squats. It appears randomly. I think it is psychological , i am being a pussy those times , the load intimidates me and i chicken out.
FWIW , I do the same thing with you to deal with it , i do a partial rep to feel the load and then carry on , im allways good to go at 2nd rep.
Lance's advice FTW , we should start with the 2nd rep! 
ah well good to know i'm not a lone lol. btw do u yawn during your workout? i do everytime.
but yeah partial reps seems good. but just that day when i was doing squat, i reallly got some focus in my just holding my breathe and screw the breathing pattern for first rep, and just go all out. the latter reps fine though.
and yeah his advice lol! makes sense indirectly though lol
