Can having my catalytic converter replaced lead to my car to start stalling?

I recently (last week) had my catalytic converter replaced on my '03 Toyota Matrix and now its stalling when I reach higher speeds. It putters when idling and then when I start to reach around 40 it slows down but doesn't completely die just coasts in a real low speed. When I restart it, it seems ok until I start to go faster again. Could this have happened when the converter was replaced? My car ran fine before I had this work done!|||Something does not adding up.Why you replaced cat?Because cat was bad.Why cat became bad?Because engine has tune up problems and raw fuel was introduced to exhaust system (incl. cat) for long period of time,basically burned up cat.Is that sound logical to you?To me ,that you're still got problems with tune up and on yours way to burn an other cat pretty soon.Check your tune up stuff first,then go and complaint about sloppy cat job.|||Luke is right on the money,your muffler is full of catalyst material from your old catalytic converter.I would take the car back and tell them to replace the muffler,at their cost.Any good mechanic would always check to see if the muffler is clogged.I would get this done as soon as possible,it can cause engine damage.|||with newer cars, you have an oxygen sensor both before, and aft of the cat, if the reading are to far out of sync with eachother, the auto will go into safe mode, since it does not know what you or the mechanic has done. these CPU's have a learning curve, which means that you must give it time to adjust to the readings it is getting. not to say that the shop may have messed up on the installation, but if they re-check it and find it to be in order, give it time, and drive it normaly until it smoothes out.|||I would take it back and have them check there work. Check to see if they got the 02sensor plugged all the way in. the wires could be lose or damaged. If they done any welding it could have damaged one of the sensors also. But Im also wondering why a car that new had to have the cat replaced anyways. Maybe you had some underlying problems before.|||well is the catalytic converter not around the exhaust area? its probably a problem that the garage have caused by the sound of it, probably best to go back and tell them the problem, could be something blocking the exhaust and not allowing all the gases to escape from the engine and cylinders causing problems