tacoseed

So ever since i bought my 94 mustang automatic transmision its been acting up and i still havent found the issue. Ive changed the spark plugs, air filter, changed the transmission filter and fluid, and o2 sensors. i currently dont have a check engine light (I just bought a obd 1 scanner to scan either way).

What it has been doing is everytime im cruising, say for instance 40mph. Out of no where, it will downshift causing the rpms to go up to about 3k-3.5k (which usually rides at about 1.5k-2.5k) or even sometimes it will let go of the tranmission completely and start to slip. Slip as in it will lose power to the tranmission and it will rev up like if its in neutral. I just want to be able to cruise and enjoy the car. I really hope its nothing to serious and its just a bad sensor or something easy to fix. Oh and by the way the car just passed 76k miles.