Hand torque and/or arrow spine are common causes of "fishtailing." However, one cuase that is often not considered is the alignment of your rest in relation to a person's natural form. "Natural form" is that individual form that no matter how hard a person tries to use a "book" form, he or she unconciously falls back into a "natural form."

Therefore, rather than running against a brick wall, try to adapt some settings to fit your form. When applying this rule on centering a rest, an exact centering of the rest might not be for you. If your rest has a windage adjustment, try dialing your rest away from the window in very small increments. If the rest-center is the cause, watch for the fishtailing to stop. If not the postion of the rest, I opt for hand-torque and/or arrow spine.