im driving my car, (its AT) and when im flooring the gas, i am revving up to 4 and when i hit about 40 mph my car rocks back and forth. i dunno how to explain this problem but sometimes i have to shift down to 3 to go further, my car normally doesnt rev so high and sometimes it doesnt shift gears i have to do it manually. anyone knnow what the problem might be? i can use any help

 

low ATF?