So I had a local tranny guy look at it. Test ride with the scanner showed some slipping in the 5-600pm range, but temps were not in the 100's.
They dropped the pan and looked inside. Didn't look too bad they said, but fluid wasn't good - tho I had it replaced not long ago.
So they did Regulator Value Kit and Apply Value. New gaskets and fluids, along with Lubegard.
Scanner now shows near zero.
So far so good - next week I'll have to drive out to the desert and test it in the high temps (fingers crossed).
'99 S10 Blazer auto 4x4 4.3 v6