2006 300C idling

I just bought a chrysler 300c 3.5L V6 and the problem is while idling on traffic most of the time the idling reaches 450rpm and at times when im braking suddenly this uccurs and then my brakes suddenly slips due to low vaccum on the master. what do you think is the problem? also my average reading on the gas is usually when i start the car is 12mpg but as i wait in traffic it goes up to only 1.2mpg what the hells wrong with my car and the dealer seems to know nothing.

any check engine light on?   and if u have low vac then check for a vac leak some where.