i had a cel that was for the egr valveso i replaced it yesterday. i reset the computer and the code came back

. my car still runs poorly. My car wont rev pst 6,000rpm (it will reach 5,000rpm fine then after it, it will very slowly g up to 6,000 and the revswill stall there) I have had this problem for little over a month now and every thing i do doesnt seem to help. i also had 2 other codes that pointed to bad precats ( i remember one code # was 420).
Any help will be greatly appreciated. I am sick of it running like this. i also get bad gas milage. Please help.