Why does my car turn off on a sudden brake and will not shift correctly?

I have a Chevy Malibu/ 6 cyl/ 3.1. Every time i have to forcely brake to avoid an accident or just not for running a red light, my car turns off. When it doesnt the RPM go less than .5 and then accelerates itseft to almost 3 and then goes back to .5 in seconds? Also when i turn left (only left) I cannot accelerate or else the RPM goes to almost 3 or 4 and will take a few seconds to shift and this was not normal as when i bought it. What can i do?