I have been having problems with my 2000 4L60e shifting from 2nd to 3rd gear on my firebird. Tranny works fine at less than full throttle shifts in that gear, and works fine in all other gears. When I go wot in 3rd the clutch holds fine. Could this be a worn out shift solenoid. I am getting ready to swap to a 4L80e, but I want to get this fixed before I sell it. Any help is appreciated.
Tranny has been built up with blue clutches, transgo shift kit and the usual.

 

I could be wrong, but you didn't say what the problem is.

 

The car won't shift from 2nd to 3rd under full throttle.