91 taurus transmission problem

I just bought a 91 taurus station wagon for my wife, when you go about 45mph the transmission will sometimes downshift to 3rd for no reason, and sometimes when you get over 55mph the transmission will hesitate, like the torque converter in locking then unlocking over and over again, then the transmission just went out, the car would just rev, i pulled over and put it in park then back in drive and it was fine, I thought it was the (vss) vehicle speed sensor going out, I changed it but nothing changed, and the check engine light is not on.........anybody know why the car is doing this?

Did you check the transmission oil level? When was it changed the last time?

I just changed the fluid and filter 2 days ago, and today I changed the (TPS) throttle positioning sensor....I think I solved it....so far its running good.