If it is an intermitent problem I really don't know. Form and grip are usually the cause of problems like this. Then, check all the obvious things like rest position, fletch clearance, tiller, cam lean, cam timing, axle to axle length, etc. If the strings/cables have stretched the bow may be out of tune.
Don't know if you have done this, but verify your centershot by walk back tuning with field points, then broadhead tune.