Rough isle. runs fine at highway speed, good acceleration.

What is your question?

It idles at about 800-900 rpm smoothly, after about 10 seconds, something changes and the rpms drop to about 550 rpm and very very rough. Very rarely will it die. It appears that the computer is telling something to open or close making it idle rough. Can not find ant vacuum leaks. Could the EGR or the idle air valve cause this? Thank you.