190e barely runs-bad cat converter?

I have a 1990 190e . The car will only go about 10 MPH and lurch along and backfire some. The catalytic converter is getting hot and smoking within a mile or two. I took it to a muffler shop and they cut both pipes in front of the catalytic converter to let it breathe and see if the cat was the problem but it still run badly so the muffler guy welded the pipes back the way they were. Any ideas would be appreciated. I did put a fuel pump relay on it a few weeks ago before this problem started and has been running fine since.