1994 Jeep Grand Cherokee runs rough after start

After you fire the motor up it runs good then after a few min. it starts to run rough at idle. You can run it uo then it smooths out and runs down the freeway good but when you let it idle it stalls. It has a new CAT a press. reg. new, and was suposed to have just been tuned up and the shop said it needed a new CAT was needed an that was why it ran rough. After changing the CAT an still haven this issue would the EGR cause this issue. thanks Ron