EGR and Cat "Check Engine" Codes

Well, I seem to have cured the "Cyl 4 Misfire" problem (below), but now am coding
1) PO401-EGR Insufficient Flow Detected
and
2) PO420-Cat System Efficient Below Threshold (Bank 1)

Should I clean/replace the EGR ($75) alone or the EGR and the EGR Sensor ($60) as well? Is it worth trying to clean the EGR Valve?

For the Cat Code, I assume I should just try a new downstream O2 sensor, or is possibly the Cat itself? The car DOES have 137K miles, but runs well (except for some surging/bucking which I attribute to the EGR). The exhaust smells "right" for a catalyst equipped car with a working cat.

I find it strange that these additional codes were NOT present when I had a Cyl 4 Misfire code, and only appeared AFTER I fixed that problem. (I am using a scanner that will retain up to 10 codes at a time)

Any ideas?

I would say that cleaning the EGR Valve would be worth the effort. After cleaning clear codes and rescan. That could take care of both problems. Your EGR, Cat and all other emmissions devices are all tied together. If one isnt working properly it can send improper data downstream. If that doesnt change anything the mreclace the sensor.