New exhaust.Now sputters!

Hey guys. My car was running fine except my girl hit a rock. She broke the front pipe so I had a new one installed, they also installed a new Cat Converter.  Now that it sounds great it is bogging out at slow speeds and the idle goes up and down...It almost took off when stopped in traffic - Good thing I was heavy on the brake.  Can someone tell me if this is a case of wrong Cat Converter or if it's a valve thing due to driving without back pressure for a while.... Thanks guys..

don't wanna sound like a dick, but why don't you take it back to where you had it fixed and ask them? obviously its related to the work that they had just done.