Why is my trans slipping ONLY when warmed up?

Thanks for the reply Tom, but that does not explain why it slips ONLY after it's warmed up, why it works OK for a short distance after shutting down for 5 mins, and why I can maintain hiway speeds with momentum. Please read the symptoms in my original post. If I could travel the 80 miles one way down the mountain -and then back up again-), I would gladly have them tell me if its an electronic control outside the trans, or if it's a mechanical problem. I've got a known good trans(parts car), but why swap them out if it's just an electronical problem that would still remain afterwards?

Cold fluid will run at a higher pressure than hot fluid. That your fluid was dark brown or black indicates that the transmission has probably been running too hot. After the rebuild install a larger transmission cooler and a ATF temperature gauge.

Good info F_O_R, thanks. Do you think the fluid and trans would cool down in just 5 mins, temporarily restoring operation? I'll check the temps next test-drive to be sure. I'm worried there might be a temperature related electronic device or signal to-or-from the pcm, causing a pressure problem. DANG ELECTRONICS! My Son wants to just swap out the trans without further research or troubleshooting....the boundless exuberance of reckless youth, or the caution of wasted labor experiences and age, right?

I'm thinking "limp mode."