dreaded po400

What is your question?

i have manually checked the egr and the engine stumbled but did not die it dropped for 800 rpm's to 400 rpm's and it wanted to die seems like, and was told my passages are ok but i'm not sure, the system seems ok and it runs fine, i replaced the egr valve and o2 sensor and have been performing several test checking all the hoses and the all checked out ok, should i turn my interest to wiring or solenoids or some thing else ?