[tccrab]

At those speeds it may be a binding that you are experiencing, but maybe not he binding that you're expecting.
Does your shifter tend to lift at higher RPM's?
Do you have Bill B.'s beam plates installed?
Are you *SURE* that you're driveline/transmission/rearend are in exact alignment?
Do you hear or feel a buzzing in the cockpit at really high rpm's (other than the engine)?
Get under the car and check to torque on the "C-Beam" bolts.

If you get enough transmission movement at speed/max RPM's, it could just be that you're shifter is lifting and binding on side of the shifter hole.

It's a WAG (Wild Azzed Guess) but it is a possibility.

TomC