When you replaced your EGR valve did you check to see if the hole in the intake was clear of carbon build up? This is the most common reason for failure of the EGR valve. You will have to use carburetor cleaner (spray can) and a piece of coat hanger wire to clean out the passage. Here is some information on testing the EGR valve. Part 1 -How to Test the GM EGR Valve (P0401, P0403, P0404, P0405, P1404, P1406).