Multple EGR codes on 2002 Mitsubishi Diamante

Hi, I have multiple EGR codes including insufficient flow, fuel is too lean, one or the catalytic convertor and one other. I've had the EGR Valve, Valve Sensor, Vacuum Control Switch, and EGR Solenoid Replaced but the light keeps coming back on. Usually the car also idles very rough. I've taken it to multiple locations with no solution. I've also had the intake manifold gasket replaced recently, along with an 02 sensor and spark plugs.

I had the catalytic convertor checked out and the guy said he doubts that's it. No vacuum leak has been detected and checked for carbon build up in the intake.

Any ideas before taking it to the dealer?

the egr valve is a rare failure on this motor. the pipes clog up with carbon and require lots of cleaning.
fuel too lean or 171-174 codes are related to vacuum leaks, low fuel pressure or mass air flow sensor.
the cat codes indicate cat failure, not o2 sensors.
you need another shop more familiar with these codes and the real diag along with the process.