MAZDA 626 1996 Attransmission The changes are thrown out

The changes are thrown out is 4 -speed automatic transmission with Overdrive but when it is heated after walking it about 20 minutes the changes to be thrown, there have even been occasions that the overdrive of flat does not enter so it is difficult to try to exceed 60km/h, in others it was neutral for a neutral for a few moments. I made three oil changes to the transmission with 5 Lliters of Mercon Oil 3 Motorcraft, in the first two I put an additive for skates for Caro Transmission, in the second one another more economical but it was not helpful. The mechanic tried it and I told me about a sporadic fault but he did not tell me that he fixed it, simply that I could put an additive, other people tell me about some bands that bring automatic transmissions and that in service workshop they are adjusted or changed, how true is this? How can I solve it?

Take a sample of the oil and verify that it does not have metal waste ..