Turn and purge will not reveal the binding of the stack bearings, remember these are thrust bearings retaining the tail rotor cuff and blade, allowing feathering - the problem occurs when the tail rotor system is rotating and the bearings are loaded. I have seen this occur in a single specific phase of flight, and no effect on any other phase.
This is not a speculative troubleshooting procedure, but one I have experienced quite a number of times.
You can change the tail rotor head, or alternately if you have a donor aircraft, pull the flapping hinge bolts and swap each entire spindle and blade assy onto your old hub, maintaining the same positions to keep the balance (the hub has very little effect on balance). You will still have to balance the tail rotor before flying, but you should get a flight out of it before lunch if you do it this way and either eliminate or confirm this as a cause.
Was this ship operational before the TGB change, or parked?