The car starts then struggles to stay on and gets shaky and then the car fully stalls when I was trying starting it again it turns on as if.

It's been 6 months due to a problem where my car has been having a rough time starting and sometimes even running, what it does is it turns on but then struggles to stay on and gets shaky, and then the car fully stalls when I was trying starting it again it turns on as nothing happened. Also, when on the road and it's around 1/4 the car. Gets very shaky and horsepower loses it even once time stalled going downhill. I've put in some Lucas injector cleaner didn't do. The only code that popped up was a p0300 but then went away. The car has more effects when turning it on, when the engine is fully warm and sometimes cold starts. It runs normally when full but when it is around 1/2 or 1/4 it runs bad. Also, when starting it sometimes smells like fuel, I've taken the car to two dealers one said it was my trans but then the other said the trans was fine and there were no problems to be found. I went to local mechanic shops and didn't want to mess with the car since there were no current check engine lights. I have a feeling it could be my battery. It looks in fair condition but I'm confused about what it could be.