What can I do to clear EGR valve code P0404?

What is your question?

Hey! I blew my last engine in my Bonneville (stupid teenager I know) and I recently dropped in a used one. It was running rough and had a CEL (check engine light). I cleaned the EGR valve with a steel brush and carburetor/throttle body cleaner as this is what I was told to do after the code was diagnosed. After I refitted the valve, the car ram much smoother for around 5 miles. But then it started running rough again. I have cleaned it twice now and this problem continues to occur. Any tips on how to fix this? Thanks!