Searching the web I found other makes of cars reporting the same - Ok when it starts, then slips when warm but if to shut off engine even for 20 seconds, it will not slip maybe for 30 sec of driving.
The root cause was the filter was clogged and by stopping the car temporarily, the fluid had some time to get through the screen. I don't think sediment comes off during the stop time, it's just time to let fluid get past. Solution is to replace the filter or in my case I cleaned the existing one.
I did a flush DIY by draining and refill 3 times. Each time about 3 qts comes out, about half of what's in there, rest is in the like the torque converter. If say half replaced each time, the contamination drops 1/2 each time - 1/2 x 1/2 x 1/2 = 1/8 of junk floating remains. Between each drain/fil I drove it around a little to mix.
Then I dropped the pan and pulled the filter. The pan was failrly clean, maybe 0.25 cubic cm of debris, small bits, plus very fine metal dust on the 3 removeable flat magnets. Cleaned the pan and magnets.
I ordered a new filter but wanted to try to clean the old one and it worked. 3 bolts and it's out into a shoebox size plastic box where I sprayed with carb cleaner inside and out. Then I was in a routine about 5 times of the following: 1) hold vertical and poor in 91% alcohol (from drug store) into the little square hole (exit port) until it comes out entrance port, then 2) take socket wrench handle and tap (not hard enough to dent it) to loosen junk off the screen, then 3) shake while tipping to dump out liquid with the debris, then 4) set it aside on a clean paper towel and funnel the box liquid back into the alcohol bottle, but don't let the debris in. Then 4) I poored the remaining debris having about a tablespoon of liquid onto a paper towel to save it for later to inspect. Then 5) wipe the plastic container and repeat.
In all about 1-2 cubic cm of stuff came out. Seems not much but definitely was root cause because when I put things back, everything was fine.
I bet a lot of transmissions get replaced or cars sold due to this problem that can be easily fixed.
I think it likely the clutch packs are ok with lots of material left.
Note - it is very important not to attempt to drive the car when it starts slipping - with fluid flow restricted and high revs, the transmission friction surfaces will be very much thermally and mechanically stressed.
A note on the throttle pressure cable - Contrary to what I read on web, I think when the cable is loose the shifts are harder rather than softer.
As for the quarts of contaminated ATF, I might try to filter it, saw some write ups, or I might let it sit a long time and let gravity work it. Fluid looked and smelled ok, no burn signs.
