I'd take a guess no matter how many flushes you do, the gearbox is farked and its in need of a rebuild
What do you mean by filthy? did it look like it had water in the oil? Any metal fragments???
2 me it sounds like the clutch packs are worn at best.
Don't forget the torque converter holds a few litres of oil and if you only emptied the pan and restarted the car the dirty oil left in the torque converter will make the new oil look filthy again. So a few flushed will be needed before it looks 100% clean.
Time is never wasted when your wasted all the time 
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