Car has 2 year old rebuilt transmission

Car has 2 year old rebuilt transmission. Notice a hard downshift from 2nd to 1st gear in first year but believed it to be the Transaxle transmission. It got worse so now after transmission warms up to full temperature it suddenly downshifts I think to 2nd or first gear I don't dare drive faster then 35-40 mph. I changed the speed sensor and that did not help. I read improper fluid level as a possible cause and thought they over filled according to dipstick so I removed quart and put in between marks when warm but problem persists same.