Well, it is a place to start. Did you ever identify the transmission? Is it the 4R55E? You mention slipping in your original post, and this could also indicate a problem with the torque converter, but you mentioned that it runs great when warm so maybe we can bypass that for now. Not sure if you have access to an oil pressure gauge but you can tell a lot from the pressures (e.g. front pump, leaking valves, etc.). I didn't, but knew from the condition of the fluid (brown, burned) that my clutches were shot. You can also tighten the bands to spec from the outside without having to open it up and see if that makes any difference.
If that doesn't help, it might be time to drop the pan and check the condition of the filter and pan. Maybe it is gunked up yet flows easier when it is warmed up explaining the "runs great" and intermittent issues. Check the condition of the valve body gasket and see if there are any apparent leaks. Also check the torque on each of the bolts according to spec.
Not sure how many miles are on the transmission, but it may be time for a valve body rebuild since you are in there. It isn't difficult to do, and I think it is quite fun to work on the brains. Pressure leaks or sticking valves can cause problems like this. Based on which transmission you have, there are threads of rebuilds to help. Might be as simple as just removing, inspecting (replacing if damaged or scored) and lubing up each valve upon reassembly. Gasket/filter kits are available at places like Napa.
Upon reassembly, use fresh fluid. Depending on which transmission/torque converter model you have, you might even be able to drain the torque converter to replace more old fluid. You can also disconnect the transmission cooler lines at the radiator (if installed) and blow out the fluid cooler with a compressor to degunk it even more. Make sure your new filter comes with an o-ring for reassembly, or use the old one if it is still good for a proper seal. Put it all back together and torque everything to spec and hopefully that will solve your problem. I think from start to finish (with all the parts and tools lined up) you can do a good valve body rebuild in just a couple of hours.
We had a slow spiral of death in ours (delayed shifts, slipping, then BAM.. no forward gears, yet reverse good and strong). Ended up being the clutch packs burned out and piston seals shot. Had to rebuild the entire transmission, but now she runs great at less than 20% of the cost for a retail rebuilt. No warranty, but at least I know I did more careful and methodical work than someone who doesn't care.
I'll help you as much as I can, and I'm sure there are many other members on here who will share some experience along the way. Cheers!