if you flushed it and it went away and is only now showing up but slightly , i would say drop the pan on the tranny , clean out the screen filter, buy and inline filter from wix and it should slowly clean out the particles that are blocking and sticking the valve. It seems to be cleaning the valves with eh new fluid as it has fresh detergent and reappears as it builds back up.
No use taking of the valve body for service , way to o much work. The easiest is just flushing again and throwing in the filter as the mush might be clean.
Also use a nice synthetic tranny fluid like Mobil 1 which i have in mine.