While a plugged cat could be causing some of the rough running issues, the Cadillac 4.x series engine uses a negative backpressure EGR valve, which isn't affected by increased exhaust backpressure (Conversely, the previous poster's Olds 98 has a positive pressure EGR, which could be adversely affected).
Mr.Seville, have you checked to see if the EGR is functioning? Start the engine and let it warm up, place your fingers under the diaphragm and increase the RPMs to about 2000. You should feel the diaphragm move under vacuum pressure.
If it doesn't move, check the vacuum hoses carefully running to the EGR solenoid, and to the EGR valve itself. Remove the valve and check the passages in the manifold and the valve, they may be plugged. The solenoids rarely fail, but if the valve works with 7 pounds of vacuum applied to it, that's the next place to look.