Windstar 98 tranny problem

While driving from my home to liberal kas after about 13 miles into a stiff wind my van suddenly shifted down i believe probably 2nd gear anyway the rpms went from 2000 to 4000 for just a few second then back to normal . thiiis has hppened a couple times. I set the Cruise on 70 coming:runaround: home and it was perfect. Any ideas on where to start looking for the problem .its full on fluid