Thank you for posting your tune.
I did not realize that you were working with the old Vortec "Black Box" PCM.
Some of the OEM transmission calibrations were not great - especially the 4L80s.
Take a look at your TCC "Normal" apply / release speeds (ECM 15200). From 12% TPS to 37% TPS the third gear apply speed is greater than the fourth gear release speed. From 56% TPS to 100 % TPS the third gear apply speed is the same as the fourth gear release speed.
If you were travelling between 37 MPH and 41 MPH during the third to fourth upshift, then TCC would lock then unlock then relock.
Some operating systems will force a TCC unlock during an upshift - not certain about the old Black Box PCM - but likely.
If you look at a graphic depiction of the table, the "lines" both intersect and overlap. It is best if all of these "lines" are separate from each other - with distinct speeds.
For smoother driveability you have several options. You could setup the TCC lockup tables to only lock the TCC in fourth gear. However, your old Black Box PCM does not feature a separate table for Tow / Haul settings - you don't have a Tow / Haul "button" like the later P01 and P59 LS engine trucks featured. This means, if you adopted the above strategy, the TCC would not lockup if you were towing in third gear.
You could develop two separate tunes - one for daily use and one for towing. Whenever you wanted to tow something, you would need to flash a different tune.
Likely your best option is to never lock the TCC in second gear, and to choose a minimum speed for the third and fourth TCC lockup - perhaps around 50 MPH - so the that TCC never locks up when you are operating in town at lower speeds.
If this was the later P01 (or P59) PCM, then it would be easy to have two tunes in one - via activating Tow / Haul mode.
As you go through your tune, you will need to look at all of the TCC speeds - Normal, Economy (likely never used), Cruise, and Hot - I recommend making all of these tables the same.
This is truly a matter of driver preference - find a setup that you like, and one that will always lockup the TCC in fourth gear at highway speeds.
2000 4L80 Vortec PCM - PT TCC speeds - chart.jpg2000 4L80 Vortec PCM - PT TCC speeds - graph.jpg