I previously had a rockland "son of tranzilla" which is essentially a rebuilt T56. 3rd gear would tend to not let me downshift from 4th to 3rd. No matter what I did a downshift into 3rd worked around 40% of the time. Anyhow, my shop changed the tranny to a RPM stage V transmission and it seems surpisingly that I still have the same problem. Downshifting into 3rd simply works when it feels like it, even when I try to give it some more pressure on the downshift 3rd gear downshift is just a chance, I try to downshift, it doesn't work and I put it back in 4th. If I the downshift into third doesn't work, I can quickly place it in second then it'll go into third, third gear upshift always works. What's the problem here? I'm getting fed up with all this bad transmission luck.

 

What's common between the two transmissions? The shifter? Is this on your '05 C6? Try to follow the shift linkage back to see what happens with it and the torque tube when 3rd is engaged from 4th..

Normally I'd say it's the 3rd gear blocker/synchro, but your trans swap should have eliminated that, as would trying to rev-match the downshift.

mike