My '00 Galant has big problems HELP!!!

I just bought a 2000 Galant. When I took it for the test drive everything was fine, I got it home and the next day it started having problems. First, when I put it into Reverse it jerks, HARD! Almost like someone has hit me from behind. Second, upon first starting out it doesn't have any power, like it doesn't want to move. Third, I was sitting in the Drive-Thru and it completely cut off. I have a warranty on the Engine, and Transmission. I want to know if anyone else has had these types of problems, and what resolved their issues. Please keep in mind that I took the car back and they did a diagnostic test and said that it wasn't the transmission, but the transmission mounts, Is there any truth in that? Because that doesn't sound quite right to me. It doesn't seem that transmission mounts would have anything to do with the loss of power to my engine. PLEASE HELP!!!!

sounds like hte car i worked on today, shifted like it was being hit, replaced the mounts, fixed :)

Thanks!