It sounds to me like your vanes may be contacting your string near your cable slide. You should ALWAYS have clearance between your vanes and string. Nothing should ever contact your vanes. If it does it can "bump" our arrow as it passes and cause it to do all kinds of squirrely things. I shoot archery league here and know that when I shoot one that really drops away for no reason its almost always because my nock turned a bit and allowed my vane to contact the string. Or, my whisker biscuit just shredded another vane.
If you don't have proper clearance try nocking an arrow then turning the shaft just a tad to increase the clearance between the vanes and string. Let me know if it helps. Cheers!
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