Problem with 4 o/d lockup, need the other Pro's

[BR3W CITY]

**This will be detailed, and long, so I'm gonna put a warning that if you don't want to read the whole thing, skip ahead to the bottom....but don't ask me something that was explained in the body **

Alright my seasoned LS brethren, we've got an issue that is causing head scratching among us. My 4th gear lockup is doing something that I can't figure out, and the data logs are just as confusing.

Backstory; Truck started its build in 09, I had a "real" job (you know, taxes, having to show up etc). Had never built an O/D auto before, always just ordered a convertor online and tossed in behind a junkyard tranny. Got a referral to All World Trans in Waukesha WI from a local bracket racer. Gave them a good core, had them do a full rebuild w o.e. parts. Debated going with the HD2 upgrade, but they didn't want to use parts they didn't source. Was told the solenoids were replaced with the o.e. master kit.
Thats all fine, but I made the first mistake, an error of omission; I allowed them to source and spec a convertor. I didn't really notice until WELL after that, that I don't have a freaking CLUE what its stall speed is.

Fast forward a month or two of build-progress. Everything is installed, truck runs and I take it for tuning. Whole setup (from cam to trans) had ~250mi of break-in. The tuner (who I'll refer to as "a$$hole") apparently locks the TCC in 3rd to boost the output #'s....like you would when trying to load up a diesel for torque spike. Noticed some weird lockup timing on the road test, and made a small adjustment to the misfire detection to prevent false knock from unlocking the convertor. Seemed good enough, and I was jacked to drive it.

Fast forward again; no normal job, spend 80% of my time building/fixing/tuning cars. Cut old tuner loose after finding tons of errors in his tune. Switch over to SD, dial it in pretty good. After 4 hours of tuning, and data logging, and tweaking, we still have this weird problem.

Problem: The shift up to 4th is just fine, but the application of o/d is really odd. It shifts to 4th, then goes to lock....applies lock but then momentarily releases it...and then re-engages locked. WTF? On the data, it looks like I'm lifting off the throttle, but I'm not.

Normally, each shift and lock point can be seen easily on the engine speed graph, by following the drop in rpm's (linear plane, unchanging load). In this case, the 4-o/d is a sudden falloff....almost like you bumped the clutch accidentally on a manual car.

So WTF is this? Nothing in our tune suggests a problem, and we've tried a base-calibrated trans table as well as our performance-changed one. There are no leaks, fluid and filter are fresh. I haven't seen anyone else describing a similar issue, at least not the way I picture it. Our only thoughts are that; something is wrong internally with the convertor, the TCC solenoid is wonky, or something in my tcc wiring is weird. None seem to fit the bill exactly, but I'm at a loss.