hey edward,

on most trucks i've dealt with that had the impco EPR , the big one about the size of a small alternator mounted on top of the engine like some of the cat GC45's had, most of the time the problem was in the EPR. And believe it or not it mostly had to do with bad LP Gas.
The age old problem of the oil separation that occurs at high heat conditions and the EPR gets oil in it and fouls the sensors up causing many problems.
As for as the CAT being the problem, i have not seen that many go bad. Maybe an O2 sensor go bad but the CAT itself no.

Before i go replacing an EPR though i would make sure the O2 sensor(s) are ok, i have seen one not read bad but still cause the engine to run rich due to poor reaction from the sensor. If you are not getting the varied ranges you would expect to see when reading a sensor while active(in closed loop), that may be part of the problem.