Did you use a genuine Volvo EGR valve? I originally fitted a Haas one from ECP but it really didn't play nicely with the car's electronics. Fitting a new genuine Volvo one solved my issue.

I found the original part number on the old unit and looked for the same online. Up until this week on returnning to work i had no message about egr.... then on Wednesday it came back. So its back to the drawing board i think.

UPDATE
I finally have my car back running as she should. I booked her in to have a TerraClean as recommended by my local garage this resolved some issues, the technician then asked where i got the EGR valve from as his software was showing some irregular readings with the temperature and flow. He recommended a Pierburg EGR so i got him to fit one £158 well spent. Since then the car has been error free and running smooth. So in conclusion don't but a cheap EGR (the first one i got was £70 online).