transmission problems?

I have a 2000 ford focus lx auto. I'm having this weird problem with what i think is my transmission. When i initially press on the brake pedal the car jerks hard. doesn't matter how much pressure i apply. This only happens in Drive, i have been switching it to Neutral before braking so that it doesnt happen. I havent been able to reproduce it in the other gears. At highway speeds it is also downshifting unexpectedly. I think its downshifting but it could also be popping into neutral. I left of the gas and it corrects itself. I think my trans is failing, which pretty much means the end of this car. I cant find anything similar on the web, most of my search results say its a warped rotor but with my neutral test that cant be the case. Can anyone tell me what this is so I can possible fix it now? I checked the trans fluid and its low while the car is off but I cant get an accurate reading while idle. My engine mounts are bad so it could be because of the movement that makes the whole dipstick wet. thanks

Update: its now popping out of gear at highway speeds