1997 Jeep Cherokee transmission problem

My jeep drives fine for about 10 miles after starting from cold. Then the check engine comes on and it starts
shifting badly. My scanner reads po700 which is a general transmission malfunction code. I'll start up the
car the next day and the check engine light will be off until I drive about another 10 miles. Then I get the same
symptoms. I have a small leak but I keep it topped off. It has been a long time since the fluid has been changed and it's brown in color. Any help would be greatly appreciated. Thank you.