EGR system replaced. Still triggering P0300 code..

[Ethan]

So I have replaced my whole EGR system. Replaced EVERY line, EGR, Solenoids, EVERYTHING. All seems to be functioning fine, running good, but still bad gas mileage. The Check engine light goes off for about 4 or 5 days then turns back on with the same EGR code. Is the next step new exhaust system? The exhaust has 238,000 miles on it, but the motor only has 50,000 on it. I'm stumped. Could it be a bad computer since I used the original one when I replaced the motor, or do you all think maybe the exhaust is just old and full of crud? Any info and or advise would be awesome. Thanks..