carnut is right, adjust the TV cable. And Katshot is right also, the TPS has a huge impact on TCC lockup speeds, so some throttle variance can have an effect.. My rule is readjust TV cable every 6 months. TPS typically only if messed with. I like to measure voltage on it at the ECM, not under the hood, as the TPS should be adjusted as the ECM "sees" it.
Also, take careful note when it unlocks, if you apply throttle slightly, does it lock up again? And then if you backoff throttle slightly back to maintain speed, does it unlock again? I had this problem, and TPS adjust and TV cable didn't affect anything.
I had an "OI" code THM200C, it ended up being an oil pressure switch inside the trans that told the ECM that the trans was NOT in 1st or 2nd. Only 2.14 geared cars and 2.41 geared cars had this "feature". I don't know the THM200 4R that well, but the 2 trans are related. It took me some time studying the FSM and the trans electrical diagrams when I figured it out. Of course, the "fix" was simply to cut the wire, allowing the ECM input to float high (logic high, typically from a 10K to 100K pullup resistor), which told the ECM it was in NOT 1st or 2nd all the time, as there is a 3rd gear ON switch, this is somewhat redundant, and posed no issues whatsoever.
So if you have the book, specifically look at your trans code (if there are any different diagrams for it) and see what else might be causing it if the TV cable adj and TPS adj doesn't cut it.