The mind is a beautiful thing, don't waste it. Let your mind, with all of its cordination sensors, tell you what to do to get your arm to catch up. As you throw your mind should tell you what the body needs to do so hand/eye coordination are in balance - it is a practice thing.
The exact same thing happened when my son started trying to get his hips moving sooner. One concrete thing that helped was to lengthen his stride which "bought" his arm time to catch up. I would often tell him to use his mind to make adjustments. It is kind of like trying to do a back flip for the first time off of a diving board. The first time you may over rotate. The next time you may under rotate. Maybe the next time you stick it. Use your mind, and adjust.
Another thing we realized was just how "athletic" an explosive delivery really is. My son had a tendency to be a bit lazy with his lead leg and foot which caused him to be "out of sorts" as he landed into foot plant - almost like the feeling of "about to fall down." As he worked at it, however, he got better - sort of like doing a back flip for the 20th time. Oh yea, always video tape yourself throwing off the mound - learn a lot that way - like whether or not you are opening up your lead shoulder prior to foot plant.