1998 Toyota Avalon PO401 Insufficient Flow

On the V6 Engine in my 1998 Avalon I recieved error code PO401 several months ago, based on Service manual repair procedures, I evauated then replaced, the EGR Valve Assembly, Position Sensor, VSV Valve, Temp Sensor, PVC Valve, and anything I could find in the chain of parts having to do with EGR. The code went away for a week or so after each replacement. ( I don't think any of the parts were defective itest them off the car and they test fine) then the code returned, If I take everything apart again and reset the code it will go away for a few days or week, but then returns and when I reset it will then return, the frequence of return increases untill I take things apart again. I am totally baffeled, any ideas!