When you say the groups shot at the farther distance were moving farther from center did you mean the center of the group was still in the approximate center or that the groups moved left or right?
If its the latter, then your bow is not exactly verticle when you are shooting which will cause the arrows to go left or right of your aiming point depending on the bow cant direction. If the group's center is still around your aiming point then all you need is more practice to tighten up your groups.
I just saw an interesting tip from Rany Ulmer last night. He said that you should be sure that your string does not come in contact with your face because if it is inconsistant then your accuracy will suffer. He also stated that you should have 'comfortable' form as long as it is consistant and not necessarily good form.