Transsmision slips after warm up

I have a 94 4Runner 4x2 with 6 cyl. Engine, automatic with 160K on it. The problem is that after the SUV warms up or after driving for a while and after a brief stop to drop some friend, the transsmision slips when I put it on the D, but it woked again if I started from 1, then 2 then D, A mechanic changed a sensor and it don´t fail so much anymore, but it still happen sometimes. I changed the trany oil and filter last February and it came out clean, no debris or metal dust. Do I need to overhaul the trany or its just another sensor thing. Thanks in advance for your help.