I had similar issues with my T/A, I had a new LS7 clutch/ Flywheel and new GM Slave installed and did not even make it home before the issue showed up again....... I replace the Mastercylinder with a new GM unit and now the car drives and shifts like new again.
But then i ran into another issue ( self induced ) i had a water leak issue, after driving in the rain the LF and sometimes LR floor board area got soaked.
I tracked it to the leak coming in at the firewall around the Mastercylinder. I folded over the gasket/seal during install.
After a little sealer was applied all is like brand new AND dry now.

Good luck.

Dave

 

I hope I got a good working GM master(my old one was working fine! with a Spec 3)..It was a REAL PITA to install!!!! I did the drill mod though..I will see this week if I have the same problem..

 

im havin the same problem even with a new master cylinder and the drill mod.. think im gonna try the slave cylinder... anyone know which one i should get ?