AFAIK, the '96 used a digital EGR, and the PCM relied upon changes in the MAP to detect EGR effectiveness and flow. Fords used a DP sensor, and they commonly fail intermittently. Your Lumina does not.
If the PCM is storing a 401, you might want to check the continutity of the three solenoid coils within the EGR, and check the EGR passages and valve pintle areas for carbon, coke, and contamination. When you get the EGR valve removed, also check the base ghasket condition.