mac, At this point a "Transmission Self diagnostic test(manual instr.)to get the all clear on solenoids/ VSS,TCM etc.,then "drain/pan drop&clean/new filter-screen/Factory fluid " be done ,then come on back.
Really gotta try this next , since "stall" seems good AND it does properly run "at times".Not too expensive ,and most would agree this is a "determinate" maintenance and probable repair step.
p.s. I've noticed many transmission valve-body bolts are LOOSE , especially "first-time-pan-drops" , gotta SNUG ALL bolts (even under filter screen) to spec - this "holds" pressure in the complex valve body circuits and "accumulators" , keeps from "bleeding out".