I certainly wish I had the answer to your question. I have a '97
Expedition that does the same thing. First and second are fine, but
3rd gear and overdrive under moderate acceleration seems to be
early. I have serviced the trans, checked the TPS, etc and have
seen no difference. It shifts and downshifts fine, other than that.
I guess I just won't worry about it until it finally starts acting
crazy. Hope you are able to find the answer.....I have had no
luck.