1999 Ford Windstar

I have a 1999 Ford Windstar w/138,000 miles A/T Transmission. The problem that I'am having is it idles high when you first start it than it idles real low. The RMP's jump up and down between 0 and 500 before it stalls. I had it put on a scope by a local parts store. They told me the code for a EGR valve came up to start w/ that so I did. Nothing changed. I really don't know whats wrong w/ it. I thinking about taking it in to be fixed but if it is a easy fix I will probably do it myself. Thank you for your time and your answer.