This same thing is happening to my rebuilt trans. It's got less than 2K on it and I got a TCC circuit stuck on code one morning. Because they couldn't replicate it again when I took it in, they wouldn't fix the problem. I'm just hoping it comes back while the rebuild is still under warranty. It did it on a cold morning, so I may have to wait until the fall.
Do you remember which component they replaced to fix the issue on yours? Or did they do a whole new trans?
L36Pro, I did the same thing as you, replaced EVERYTHING hoping it was an electrical problem causing my bad trans symptoms. Ended up being the trans.
The EPC and TCC solenoids go out all the time in these trans's. It's been shown that the temps are very high in our transmissions, which degrades the fluid at a faster than normal rate, so if you follow the manufacturer's intervals for changing the fluid, it's sometimes not often enough. To prevent the problem once you replace the faulty components, you can add in an external trans. fluid cooler, which will keep the temps down where they should be. The stock cooler is inside the radiator tank closest to the battery, which saved GM money, but doesn't cool enough.
However, if it was me, I'd just throw out the trans and get a TEP built one. If you're gonna have someone else rebuild it, the costs are almost the same.