I have the same problem, sorry to hijack but maybe this will shed some additional light...
I just swapped boxes and I have similar problems, however left of center and turning left it seems almost sticky if I pulse my steering imputs (stopping and starting). THis only happens once the fluid is warm. To the right it works better but is still slightly noticible. I'm using what I suppose amounts to store brand power steering fluid which from reading the bottle is petroleum based like the manual calls for.
Curious though as to why overfilling would cause a problem-not that I doubt, just curious. Hope the additional info and angle help