Sounds grip or centershot related to me. Make sure you hold the bow with the same torque free (neutral) grip everytime and if you haven't tried a wrist sling and a open or open relaxed (fingers just laying gently on the front of the riser), then you might want to try that. This is assuming the bow fits you and the arrows are properly matched to the bow (which is first and foremost).

If that doesn't help, you might try to download Martin's tuning manual and work with the group tuning section on horizontal disturbances. You'll be shooting at a vertical line and making adjustments to see what makes an improvement. Good luck.

-Chief

http://www.martinarchery.com/manual/comp1.html -if you shoot a compound bow.