A friend's car had a similar problem. I centered the shifter and checked the torque on the u-joint pinch bolts, which were tight. A test drive revealed no joy. The problem turned out to be one of the shift shaft's splined ends was moving in the u-joint even though the pinch bolt was very tight. I recentered the shifter and tightened the carp out of the offending pinch bolt. Problem solved.
John