Problem with EGR Valve
 

My 2006 Rendezvous was stalling at low rpms. Stop lights, turning corners, etc. OBDII code P0405. After checking all wires coming into the EGR, we replaced it. Car did the same thing. Took it to a licensed shop and they said it would have to go to the dealer to have the computer reset. He stated the computer was not reading the new EGR valve. I have called 2 dealerships and they seemed clueless. Does anyone have any tricks to reset the computer or any ideas as if this is actually true, or do I have another issue on my hands as well? Thank you so much for reading this question, we have been working on this problem for weeks!!! Very close to using the car for target practice! Probably would, but it is a very solid, great running car other than this one aggravating issue!

Begging for help!!!
 

Please, if this is Buick forum, someone out there has to have heard/dealt with this issue. At this point I feel like I have a piece of junk sitting in my garage!!!