I presume you are referring to the flexible part that connects the tranny rod to the 1st shifter rod, and not the trunnion, nor the linkage inside the car. Yes?
...and that is slips fore and aft on the spline?
...and torquing down on each of the two bolts has not stopped the movement?
On my car, the turn-bucket squeezes the spline so tight that it does not move even without the bolts!
This may sound odd, but if it slips down the spline, then you could spray paint (&dry) the spline, then bolt together again. The paint should cause enough friction to stop the slippage. Maybe another layer of paint after it is bolted together.
If this was not your problem, let me know, as I recently R&R'ed my linkage, and have other experiences.
R